Polylactic acid resin, textile products obtained therefrom, and processes for producing textile products

ABSTRACT

A polylactic acid resin suitable for use especially in textile products; textile products obtained from the resin as a raw material (a fiber, multifilament, monofilament, staple, false-twist yarn, long-fiber nonwoven fabric, etc.); and processes for producing these textile products. The polylactic acid resin is a resin consisting mainly of a polylactic acid and is characterized in that it is linear, has an L-isomer content of 95 mol % or higher, an Sn content of 30 ppm or lower, a monomer content of 0.50 wt. % or lower, and has a relative viscosity of 2.7 to 3.9 or has a weight-average molecular weight of 120,000 to 220,000 and a number-average molecular weight of 60,000 to 110,000. Each of the textile products comprises the polylactic acid resin as the main material. The textile products each comprises a polylactic acid that is excellent in processability and excellent fiber properties. The free textile products are problems in practical use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional under 35 U.S.C. §120 of prior U.S.patent application Ser. No. 10/018,732, filed on Mar. 8, 2002 now U.S.Pat. No. 7,445,841 as a national phase entry of InternationalApplication No. PCT/JP00/04000, filed on Jun. 19, 2000, which claims thebenefit of Japanese Patent Application Nos.: 11/172414, filed on Jun.18, 1999; 11/205,836, filed on Jul. 21, 1999; 11/205,838, filed on Jul.21, 1999; 11/210,370, filed on Jul. 26, 1999; 11/216,585, filed on Jul.30, 1999; 11/259,914, filed on Sep. 14, 1999; 11/264,727, filed on Sep.20, 1999; 11/273,086, filed on Sep. 27, 1999; and 2000/609, filed onJan. 6, 2000, all of which are incorporated herein by reference in theirentireties.

FIELD OF THE INVENTION

The present invention relates to a resin mainly comprising polylacticacid and textile products using the resin as a starting material, andprocesses for producing the textile products.

BACKGROUND ART

The most widely used textile materials today include synthetic resinssuch as polyesters represented by polyethylene terephthalate andpolyamides represented by 6-nylon and 66-nylon.

While these synthetic resins are advantageous in their capability ofcheap mass production, they involve some problems related to theirdisposal. The textile made of such synthetic resins can be hardlydecomposed in the natural environment, and high heat of combustion isgenerated by incineration.

Under these situations, use of biodegradable synthetic resins such aspolycaprolactone and polylactic acid for textiles have been proposed.Although these resins are excellent in biodegradability, they are stillnot suitable for practical applications as compared with non-degradablesynthetic resins such as polyethylene terephthalate and nylon that havebeen widely used.

These problems are poor process throughput during the producing process(spinning, drawing, false twisting and the like), inferior propertiessuch as tensile strength and elongation percentage of the textileproducts obtained as compared with conventional synthetic fibers.

The inventors of the present invention have made intensive survey on thephysical and chemical properties of polylactic acid, and haveinvestigated polylactic acid resins particularly suitable for use in thetextile products. We have also found polylactic acid textile productsbeing excellent in productivity and having favorable properties by usingpolylactic acid having selected properties, and a process for producingthe textile products. The object of the present invention is to providepractically acceptable textile products comprising polylactic acidhaving excellent properties for use in textiles with high productivity.

SUMMARY OF THE INVENTION

The object as hitherto described is attained by a polylactic acid resinmainly comprising linear polylactic acid comprising 95 mol % or more ofthe L-isomer and containing 0 or 30 ppm or less of tin(Sn) and 0 or 0.5%by weight or less of monomer content with a relative viscosity ηrel of2.7 to 3.9, and a polylactic acid resin mainly comprising linearpolylactic acid comprising 95 mol % or more of the L-isomer andcontaining 0 or 30 ppm or less of Sn and 0 or 0.5% by weight or less ofmonomer content with a weight average molecular weight Mw of 120,000 to220,000 and number average molecular weight Mn of 60,000 to 110,000. Thepresent invention also provides a textile product mainly using thepolylactic acid resin as a starting material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates the drawing process according to thepresent invention; and

FIG. 2 schematically illustrates the conventional drawing process.

REFERENCE NUMERALS

-   -   1 roller heater    -   2 roller heater    -   10 non-drawn fiber    -   20 drawn fiber    -   21 roller heater    -   22 plate heater    -   23 cold roller

DETAILED DESCRIPTION OF THE INVENTION

The polylactic acid resin according to the present invention, fiberthereof, and the process for producing them will be described first.

The polylactic acid resin according to the present invention include (1)a polylactic acid resin mainly comprising linear polylactic acidcomprising 95 mol % or more of the L-isomer and containing 0 or 30 ppmor less of Sn and 0 or 0.5% by weight or less of monomer content with arelative viscosity ηrel of 2.7 to 3.9, and (2) a polylactic acid resinmainly comprising linear polylactic acid comprising 95 mol % or more ofthe L-isomer and containing 0 or 30 ppm or less of Sn and 0 or 0.5% byweight or less of monomer content with a weight average molecular weightMw of 120,000 to 220,000 and number average molecular weight Mn of60,000 to 110,000. The polylactic acid fiber according to the presentinvention and the producing process thereof comprise the followingelements: (3) a polylactic acid fiber comprising the polylactic acidresin in (1) or (2) above; and (4) a process for producing thepolylactic acid fiber by melt-spinning using polylactic acid in (1) or(2).

Polylactic acid to be used in the present invention has a linearstructure, or substantially has no branched structure. A small amount ofbranching agent has been added during polymerization of polylactic acidin order to improve melt viscosity and degree of polymerization in theformer proposal. However, it was confirmed by the inventors of thepresent invention that the branched structure of the starting resinmaterial for producing the polylactic acid fiber has a far more negativeeffect on spinning work efficiency as compared with production ofconventional polyester fibers. In other words, even a small amount ofthe branched structure adversely affect spinning work efficiency ofpolylactic acid, besides the fiber obtained has a low tensile strength.

For excluding the branched structure, it is recommended that chemicalsthat causes branched structures in the polymer material, for examplethree valent or four valent alcohols and carboxylic acids, are not usedat all. When these chemicals are forced to use for some other reasons,the amount of use should be restricted within a range as small aspossible so that spinning work efficiency is not adversely affected.

Although polylactic acid used in the present invention is derived from astarting material such as L-lactic acid or D-lactic acid, or L-lactideor D-lactide as a dimer of lactic acid, or mesolactide, it is essentialthat the proportion of the L-isomer is 95 mol % or more. This is becauseincreased proportion of the D-isomer makes the polymer amorphous andcrystal orientation is not advanced in the spinning and drawing process,thereby deteriorating the properties of the fiber obtained. Inparticular, the tensile strength remarkably decreases with excesscontraction ratio in boiling water to make the fiber to be practicallyinapplicable.

Polylactic acid to be used in the present invention is required tocontain 0 or 30 ppm or less, preferably 0 or 20 ppm or less, of Sncontent in the polymer. While the Sn content based catalyst is used as apolymerization catalyst of polylactic acid, a content of more than 30ppm causes depolymerization during the spinning process to allow thefiltration pressure at the nozzle to increase in a short period of time,thereby remarkably decreasing spinning work efficiency.

For decreasing the Sn content, the amount of use for polymerization maybe decreased, or chips may be washed with an appropriate solvent.

The polylactic acid to be used in the present invention contains 0.5% byweight, preferably 0.3% by weight or less and particularly 0 or 0.2% byweight or less, of monomers. The monomer as defined in the presentinvention is referred to the component having a molecular weight of1,000 or less as calculated from a GPC assay. A content of the monomerof more than 0.5% by weight causes remarkable decrease of workefficiency, because heat resistance of polylactic acid decreases due toheat decomposition of the monomer component.

For reducing the monomer content in polylactic acid, unreacted monomersare removed by evacuation of the reaction vessel immediately beforecompleting the polymerization reaction, polymerized chips are washedwith an appropriate solvent, or polylactic acid is produced by a solidphase polymerization.

Polylactic acid to be used in the present invention preferably has aweight average molecular weight Mw of 120,000 to 220,000 and numberaverage molecular weight Mn of 60,000 to 110,000. While the molecularweight in this range afford excellent spinning ability and sufficienttensile strength, the molecular weight out of this range causes largedecrease of the molecular weight during sinning to fail in obtaining asufficient tensile strength.

Polylactic acid to be used in the present invention has a relativeviscosity ηrel of 2.7 to 3.9. The relative viscosity of lower than thisrange causes to reduce heat resistance of the polymer and to fail inobtaining a sufficient strength, while the relative viscosity of higherthan this range requires an elevated spinning temperature to causeheat-degradation during the spinning process.

The relative viscosity having a lower reduction ratio during thespinning process is favorable and the preferable reduction ratio ofrelative viscosity is 7% or less for spinning multifilaments. Areduction ratio of 7% or less substantially results in no decompositionof the polymer during spinning, give rise to good spinning abilitywithout arising broken fibers during spinning, and enabling particularlyhigh tensile strength in the drawing process.

It is preferable for practical production that the fiber produced has atensile strength of 3.5 cN/dtex or more.

Examples of the polylactic acid fiber according to the present inventioninclude multifilament, staple fiber, spun-bond, monofilament and flatyarn.

The fiber according to the present invention can be obtained bymelt-spinning process known in the art.

A biodegradable fiber excellent in work efficiency and properties of thetextile may be obtained by producing the polylactic acid fiber using theresin according to the present invention. According to the process ofthe present invention, the polylactic acid fiber having physicalproperties such as tensile strength, drawing ratio and contraction ratioin boiling water comparable to conventional polyester and nylon fiberscan be obtained, wherein the fiber is excellent in heat resistancewithout decreasing spinning ability, the spinning nozzle has asufficiently long service life, and the fibers are free from breakageand fluffs.

The present invention will be described in more detail with reference toexamples. Analysis of the properties of the polymer will be describedfirst.

(Molecular Weight/Monomer Content)

Samples were dissolved in chloroform in a concentration of 10 mg/mL, andMw and Mn were measured by the GPC assay using Waters LC Model I Plusequipped with a RI detector. Polystyrene was used as a standardsubstance of the molecular weight.

The proportions of the monomer in the polymer was calculated from theproportion of the component having a molecular weight of 1,000 or less.

(Relative Viscosity)

The samples were dissolved in a mixed solvent ofphenol/tetrachloroethane=60/40 (in weight ratio) in a concentration of 1g/dL, and the relative viscosity was measured at 20° C. using aUbberohde viscosity tube.

(Sn Content)

The sample (0.5 g) was ashing by a wet process using sulfuricacid/nitric acid. The ashing sample was diluted with water to give a 50mL sample solution, and the Sn content was measured using an ICPemission spectrometer SRS 1500VR made by Seiko Instruments Inc.

(Heat Stability)

The temperature at a mass reduction of the polymer of 5% was measured asTG (5%) using Seiko Instruments Inc TG/DTA 220U.

Spinning work efficiency and fiber properties were measured andevaluated as follows.

(Evaluation of Spinning Ability—1)

A 7-days' continuous spinning was performed by melt spinning. Incidenceof broken fibers were evaluated in three steps (A, B and C) below:

-   -   A: zero time of broken fiber in 7 days;    -   B: one to two times of broken fiber in 7 days; and    -   C: three or more times of broken fiber in 7 days.        (Evaluation of Spinning Ability—2)

Service life of the spinning nozzle was evaluated in terms of days whenthe spinning nozzle was forced to change by increment of filtrationpressure during the 7-days' continuous spinning.

(Evaluation of Spinning Ability—3)

Incidence of broken fibers in the drawing process was evaluated in threesteps of A, B and C:

-   -   A: zero time of broken fiber in 7 days;    -   B: one to two times of broken fiber in 7 days; and    -   C: three or more times of broken fiber in 7 days.        (Measurements of Tensile Strength and Elongation Percentage)

Using a tensile strength tester manufactured by Shimadzu Co., a tensiletest was performed at a speed of 20 cm/min using a sample with a lengthof 20 cm, and the tensile strength and elongation percentage wasmeasured from the ultimate strength and ultimate elongation percentage,respectively.

(Contraction Ratio in Boiling Water)

A 200 mg weight was hanged to a sample with an initial length of 50 cm,and the sample was immersed in boiling water for 15 minutes followed bydrying in the air for 5 minutes. The contraction ratio in boiling waterwas determined by the following equation:Contraction ratio (%)=(initial sample length−sample length aftercontraction)/initial sample length×100(Fluffs)

Incidence of fluffs after reeling the drawn fiber was evaluated by thefollowing two steps (o and x).

-   -   o: no incidence of fluffs; and    -   x: incidence of fluffs.        (Productivity of Filament)

Total evaluations of the filament was made in three steps of A, B and Cby considering the evaluation of spinning ability 1, 2 and 3, andincidence of fluffs:

-   -   A: very good    -   B: good    -   C: poor        (Rate of Decrease of Viscosity During Spinning)

The relative viscosity (ηrel) of the filament extruded out of thespinning nozzle was measured, and the rate of decrease of viscosityduring spinning was determined from the following equation. Theresidence time of the molten polymer in this example was about 10minutes.The rate of decrease of viscosity during spinning (%)=[(relativeviscosity of the polymer−relative viscosity of the filament)/relativeviscosity of the polymer]×100(Polymerization of the Polymer)

L-lactide or D-lactide as a starting material was polymerized topolylactic acid using tin octylate as a polymerization catalyst byconventional polymerizing step. Polymerization was also carried out byadding 0.1 mol % of trimellitic acid as a cross-link agent (ComparativeExample 10). While the polymer obtained was subsequently subjected tosolid state polymerization at 135° C. to reduce the amount of theresidual monomers, the solid state polymerization was omitted in a partof the samples for comparison.

(Spinning)

Filaments of 84 dtex/24 f were obtained by a conventional filamentprocess of spinning and drawing by extruding the molten resin in the airthrough a spinning nozzle with a spinning hole diameter of 0.25 mm andnumber of spinning holes of 24. The spinning test was continued for 7days to evaluate spinning ability, service life of the nozzle andincidence of fluffs during drawing.

Examples 1-1 to 1-2, and Comparative Examples 1-1 to 1-5

Table 1-1 shows the changes of spinning ability, service life of thenozzle and incidence of fluffs during drawing when the content of Sn inthe polymer is changed, and the results of the quality of the fiber.

In Comparative Examples 1-1 to 1-3, the polymer had been depolymerizedduring spinning due to particularly large content of Sn (the amount ofthe residual catalyst). Consequently, the viscosity was largelydecreased during the spinning step to make it very difficult to spin. Inaddition, the service life of the nozzle was a short as one day, quitelarge number of fluffs had generated during the spinning step due tolarge rate of decrease of viscosity during the drawing step, and thefiber obtained had a quite poor tensile strength of 2.6 cN/dtex or lessto make it impossible to use the fiber for practical purposes.

While the rate of decrease of viscosity during spinning was improved to17.6% in Comparative Example 1-4, the service life of the nozzle was asshort as three days. Although incidence of fluffs during drawing wasalso improved, the fiber was inappropriate for practical uses since apractical tensile strength of the fiber of 3.5 cN/dtex was not attained.

The service life of the nozzle was increased to six days and the tensilestrength of the fiber satisfied the practical level of 3.5 cN/dtex ormore in Comparative Example 1-5, since the rate of decrease of viscosityduring spinning was improved to 12.3%. However, improvement of incidenceof fluffs was yet insufficient because the resin contained as much Sncontent as 35 ppm.

In Examples 1-1 and 1-2, the rate of decrease of viscosity was as smallas 5.0%, and spinning ability, service life of the nozzle and incidenceof fluffs during drawing were very excellent, since the content of Sn inthe resin was 50 ppm or less. The tensile strength of the filamentobtained was also excellent showing a level of 4.0 cN/dtex or more.Particularly, since the rate of decrease of viscosity during spinningwas 7% or less, the degree of polymer degradation during the spinningprocess was small with no incidence of break of fibers during thespinning process, enabling good spinning ability to be obtained as aresult of high tensile strength during the drawing process.

TABLE 1-1 Comparative Example Example No. 1- 1 2 3 4 5 1 2 Sn Content(ppm) 824 412 82 62 35 26 17 Relative Viscosity of 2.96 2.95 2.97 2.943.00 2.93 2.98 Polymer (ηrel) Monomer Content (% 0.26 0.23 0.25 0.240.26 0.26 0.25 by weight) Branched Structure Non Non Non Non Non Non NonL-isomer (mol %) 96.4 97.0 96.6 95.5 97.1 97.8 96.4 Spinning Temperature230 230 230 230 230 230 230 (° C.) Rate of Decrease of 73.6 64.3 52.317.6 12.3 5.0 3.6 Viscosity during Spinning (%) Spinning Ability 1 C C CC~B B A A Spinning Ability 2 1 1 1 3 6 ≧7 ≧7 Spinning Ability 3 C C CC~B B A A Fluffs X X X ◯ X~◯ ◯ ◯ Productivity of Filament C C C C~B B AA Tensile Strength 1.78 1.87 2.23 3.14 3.76 4.38 4.53 (cN/dtex)Elongation (%) 26.3 27.3 28.3 28.6 30.3 29.3 28.6 Contraction Ratio in13.4 15.6 14.6 15.3 11.6 11.2 10.5 Boiling Water (%)

Examples 1-3 to 1-5, and Comparative Examples 1-6 to 1-9

Tables 1-2 and 1-3 show the changes of spinning ability, service life ofthe nozzle and incidence of fluffs during drawing when the monomercontent in the polymer is changed and the results of the quality of thefiber.

In Comparative Examples 1-6 to 1-8, the resin was heat-decomposed duringspinning due to particularly large content of the monomer in thepolymer. Spinning was quite difficult due to large decrease of theviscosity of the polymer during spinning, the service life of the nozzlewas only one day, and a large quantity of fluffs was generated in thedrawing process. The filament obtained had a poor fiber quality with atensile strength of less than 3.5 cN/dtex to make the filament to bepractically inapplicable.

The monomer content was also large in Comparative Example 1-9, and theresin was inadequate for practical use since the service life of thenozzle was as short as five days.

TABLE 1-2 Comparative Example No. 1- □ □ □ □ Monomer Content (% byweight) 10.2 5.76 3.46 0.98 Relative Viscosity of Polymer 2.96 2.89 2.923.02 (ηrel) Branched Structure Non Non Non Non Sn Content (ppm) 18 19 1817 L-isomer (mol %) 95.4 96.0 95.6 96.5 Spinning Temperature (° C.) 230230 230 230 Rate of Decrease of Viscosity 25 20 15 10 during Spinning(%) Spinning Ability 1 C C C B Spinning Ability 2 1 1 2 5 SpinningAbility 3 C C C B Fluffs X X X X~◯ Productivity of Filament C C C C~BTensile Strength (cN/dtex) 2.67 2.75 3.29 3.25 Elongation (%) 26.8 26.427.9 28.9 Contraction Ratio in Boiling Water 12.4 14.6 13.2 12.3 (%)

The rate of decrease of viscosity during spinning was improved to 5% orless in Examples 1-3 to 1-5, since heat decomposition could besuppressed by reducing the monomer content to 0.5% by weight or less.Spinning ability, service life of the nozzle and incidence of fluffsduring drawing were also favorable in addition to high tensile strengthof the filament obtained of 4.0 cN/dtex or more.

TABLE 1-3 Example No. 1- 3 4 5 Monomer Content (% by weight) 0.47 0.260.15 Relative Viscosity of Polymer (ηrel) 2.96 2.98 3.02 BranchedStructure Non Non Non Sn Content (ppm) 19 21 16 L-isomer (mol %) 96.898.4 98.4 Spinning Temperature (° C.) 230 230 230 Rate of Decrease ofViscosity during 5 2 1.5 Spinning (%) Spinning Ability 1 A A A SpinningAbility 2 ≧7 ≧7 ≧7 Spinning Ability 3 A A A Fluffs ◯ ◯ ◯ Productivity ofFilament A A A Tensile Strength (cN/dtex) 4.33 4.58 4.68 Elongation (%)30.3 29.6 30.6 Contraction Ratio in Boiling Water (%) 10.2 10.9 9.8

Examples 1-6 to 1-7, and Comparative Examples 1-10 to 1-14

Tables 1-4 and 1-5 show the result of spinning with respect to changesof the proportion of L-isomer, presence/absence of the branchedstructure, and the molecular weight of the polymer and relativeviscosity.

Although the polymer in Example 1-6 has similar properties to thepolymer in Comparative Example 1-10 except the presence or absence ofthe branched structure, the polymer in Comparative Example 1-10 havingthe branched structure has somewhat poor spinning ability whilegenerating fluffs during drawing, and the tensile strength of the fiberobtained in the comparative example is lower than 3.5 cN/dtex ascompared with that of the fiber without any branches. Accordingly, thefiber in Comparative Example 1-10 was practically inapplicable.

Crystal orientation is not advanced during spinning and drawing in thefiber in Comparative Example 1-14 (Table 1-5) containing less than 95mol % or less of the L-isomer due to the decreased content of theL-isomer. The tensile strength thereof was less than 3.5 cN/dtex with acontraction ratio in boiling water of 30% or more. Therefore, thefilament was practically inapplicable due to poor dimensional stabilityin usual wove and knit processing.

TABLE 1-4 Example No. 1- □ □ Branched Structure Non Non L-isomer (mol %)98.7 96.0 Relative Viscosity of Polymer (ηrel) 3.02 3.68 Molecularweight (Mw) 14.6 × 10^(□) 19.5 × 10^(□) Molecular weight (Mn)  7.2 ×10^(□)  9.4 × 10^(□) Sn Content (ppm) 18 17 Monomer Content (% byweight) 0.27 0.27 Spinning Temperature (° C.) 230 230 Rate of Decreaseof Viscosity during 3 4 Spinning (%) Spinning Ability 1 A A SpinningAbility 2 ≧7 ≧7 Spinning Ability 3 A A Fluffs ◯ ◯ Productivity ofFilament A A Tensile Strength (cN/dtex) 4.43 4.38 Elongation (%) 30.330.8 Contraction Ratio in Boiling Water (%) 9.8 14.8

TABLE 1-5 Comparative Example No. 1- 10 11 12 13 14 Branched Yes Non NonYes Non Structure L-isomer (mol %) 99.0 96.4 97.0 98.7 92.6 Relative3.04 2.58 4.02 4.03 3.02 Viscosity of Polymer (ηrel) Molecular 14.8 ×10⁴ 10.2 × 10⁴ 23.8 × 10⁴ 24.0 × 10⁴ 14.5 × 10⁴ weight (Mw) Molecular 7.6 × 10⁴  5.4 × 10⁴ 12.1 × 10⁴ 12.4 × 10⁴  7.1 × 10⁴ weight (Mn) SnContent (ppm) 19 18 20 18 21 Monomer Content 0.26 0.26 0.25 0.24 0.27 (%by weight) Spinning 230 230 245 245 230 Temperature (° C.) Rate ofDecrease 6 8 15 20 3 of Viscosity during Spinning (%) Spinning Ability 1B B C C A Spinning Ability 2 4 4 5 3 ≧7 Spinning Ability 3 B C C C BFluffs X X X X X Productivity of Filament C B C C B Tensile Strength3.51 3.37 3.55 3.41 2.67 (cN/dtex) Elongation (%) 29.6 28.7 30.2 29.830.3 Contraction 10.2 10.1 9.7 10.2 30.5 Ratio in Boiling Water (%)

The polymer in Comparative Example 1-11 had so low molecular weight andrelative viscosity that spinning and drawing ability become poor with alow tensile strength of less than 3.5 cN/dtex. In contrast, the polymersin Comparative Examples 1-12 and 1-13 had so high molecular weight andrelative viscosity that an elevated spinning temperature was required.However, the rate of decrease of viscosity during spinning was increasedto 15% by increasing the spinning temperature to deteriorate spinningand drawing ability with incidence of fluffs during drawing, therebymaking the fiber practically inapplicable.

(Multifilament)

The multifilament according to the present invention will be describedhereinafter.

The multifilament according to the present invention can comprises theone constitution element of the following two constitution elements ofthe invention:

(5) a multifilament comprising a linear polylactic acid containing 98mol % or more of the L-isomer, 0 or 30 ppm or less of Sn content and 0or 0.5% by weight or less of monomers with a relative viscosity of 2.7to 3.9; and

(6) a multifilament comprising a linear polylactic acid containing 98mol % or more of the L-isomer, 0 or 30 ppm or less of Sn and 0 or 0.5%by weight or less of monomers with Mw of 120,000 to 220,000 and Mn of60,000 to 110,000.

The preferable embodiments of (5) and (6), comprise the followingfeatures:

(7) a multifilament having a tensile strength of 3.9 cN/dtex or more,contraction ratio in boiling water of 12% or less, birefringence (Δn) of0.025 or more and peak temperature of thermal stress of 85° C. or more;and

(8) a multifilament according to the feature (5) having an inert contentof 3.0% or less and contraction ratio in boiling water of 12% or less.Inert as used herein means fibers having irregular linear density.

The process for producing the multifilament according to the presentinvention comprises the following two features:

(9) a process for producing the polylactic acid multifilament using thepolylactic acid according to the features (5) or (6) comprising thesteps of spinning at a speed of 3,000 m/min or more to 4,500 m/min orless, drawing at a draw magnification factor of 1.3 or more at a drawtemperature of 100 to 125° C., and heat-setting at 125 to 150° C.; and

(10) a process for producing the polylactic acid multifilament using thepolylactic acid according to the features (5) comprising the steps ofdrawing between the roller heaters (1) and (2), and heat-setting at theroller heater (2).

In the conventional method, the polylactic acid biodegradable fiber ismanufactured by spinning at a low speed of 3,000 m/min or less followedby drawing. Although Japanese Patent Application Laid-open No. 7-216646and 7-133569 disclose, for example, a producing method in which anon-drawn polylactic acid fiber spun at a speed of 1000 m/min or less isreeled and an orientation fiber is obtained in the drawing step,copolymerization of polyethylene glycol is necessary in the processdisclosed above.

However, work efficiency of the producing process can be hardly improvedby the processes described above, and it was impossible to obtainphysical and chemical properties and work efficiency comparable to thefibers made of conventional (non-biodegradable) synthetic resins.

The inventors of the present invention have strictly surveyed thechemical and physical properties of polylactic acid as a startingmaterial of the fiber, and have succeeded in providing a polylactic acidmultifilament having such properties as tensile strength, elongationpercentage and contraction ratio in boiling water comparable topolyester and nylon fibers, as well as being compatible topost-processing such as weaving, knitting and dyeing as in the polyesterand nylon fibers, by using polylactic acid having selected propertiesand by investigating the spinning and drawing steps.

Polylactic acid to be used in the present invention has a linearstructure, or substantially has no branched structure. It has beenproposed in the former proposal to add a small amount of branching agentin polymerization of polylactic acid in order to improve melt viscosityand degree of polymerization. However, it was confirmed by the inventorsof the present invention that the branched structure of the resinmaterial far more negatively affects work efficiency of spinning ascompared with conventional polyester fibers in producing the polylacticacid fiber. Polylactic acid containing even a small amount of thebranched structure exhibits lower tensile strength than polylactic acidcontaining no branched structure.

For excluding the branched structure, it is recommended not to use anyagents such as trivalent or quadrivalent alcohol and carboxylic acidsthat arises the branched structure in the polymer material. When thecomponents having such structure as described above are forced to usefor some reasons, the amount of use should be restricted within aminimum essential quantity that does not affect work efficiency ofspinning such as break of fibers.

While polylactic acid to be used in the present invention comprisesL-lactic acid or D-lactic acid, or L-lactide or D-lactide as a dimer oflactic acid, it is crucial that lactic acid comprises 98 mol % or moreof the L-isomer. This is because the polymer becomes amorphous when theproportion of the D-isomer increases and crystal orientation isinhibited in the spinning and drawing steps, thereby making theproperties of the fiber obtained poor. In particular, the tensilestrength is extremely degraded while excessively increasing thecontraction ratio in boiling water to make practical application of thefiber impossible.

Polylactic acid to be used in the present invention contains 0 or 30 ppmor less, preferably 0 or 20 ppm or less, of Sn. While Sn base catalystused as a polymerization catalyst of polylactic acid, a residual amountof Sn of over 30 ppm causes depolymerization during spinning to bringabout rapid increase of the nozzle pressure and extremely decreased workefficiency of spinning.

In order to reduce the content of Sn, the amount of Sn used forpolymerization is reduced to be as small as possible, or the chip iswashed with an appropriate solvent.

The monomer content in the polylactic acid to be used in the presentinvention is 0.5% by weight or less, preferably 0.3% by weight or lessand in particular 0 or 0.2% by weight or less. The monomer as defined inthe present invention refers to the component with a molecular weight of1,000 or less as measured by the GPC analysis. Work efficiency of thefiber decreases due to occurrence of break of fibers in the spinning anddrawing steps, when the monomer content exceeds 0.5% by weight. This isbecause the monomer component is decomposed by heat to decrease heatresistance of polylactic acid.

Unreacted monomers may be removed by evacuating the reaction vessel justbefore completing the polymerization reaction, polymerized chips may bewashed with an appropriate liquid, or polylactic acid is synthesized bysolid phase polymerization in order to reduce the content of monomers inpolylactic acid.

Polylactic acid to be used in the present invention preferably has aweight average molecular weight Mw of 120,000 to 220,000, morepreferably 130,000 to 160,000. Polylactic acid to be used in the presentinvention preferably also has a number average molecular weight Mn of60,000 to 110,000, more preferably 70,000 to 90,000. While a molecularweight in this range allows an excellent spinning ability and sufficienttensile strength to be obtained, a sufficiently high tensile strengthcannot be obtained at a molecular weight as low as out of this rangebecause large decrease of the molecular weight.

Polylactic acid to be used in the present invention has a relativeviscosity of 2.7 to 3.9. A relative viscosity lower than this rangemakes heat resistance of the polymer poor, while a relative viscosityhigher than this range requires the spinning temperature to be increasedto cause heat degradation during spinning. The preferable relativeviscosity is in the range of 2.9 to 3.3.

The lower the reduction ratio of the relative viscosity of themultifilament during spinning is preferable, and the reduction ratio is,for example, preferably 0 or 7% or less relative to the polymer. Thereduction ratio of 0 or 7% or less substantially causes no decompositionof the polymer during spinning, makes spinning ability good withoutarising break of fibers during spinning, and allows the tensile strengthin the drawing step to be particularly high.

The multifilament according to the present invention preferably has atensile strength of 4.0 cN/dtex or more, because no break of fibersoccurs during each processing step. A birefringence of 0.030 or more isrequired for increasing the tensile strength to 4.0 cN/dtex or more.

The peak temperature of thermal stress of the multifilament ispreferably 85° C. or more, more preferably 90° C. or more, in order toprevent dyeing from being fatigued when the multifilament is dyed underan atmospheric pressure. A peak temperature of thermal stress of 85° C.or more is preferable since the degree of fatigue of the dye is reduced.

The multifilament preferably has an inert content of 3% or less in themultifilament according to the present invention comprising linearpolylactic acid containing 98 mol % or more of the L-isomer, 0 or 30 ppmor less of Sn and 0 or 0.5% by weight or less of monomers with arelative viscosity of 2.7 to 3.9. An inert content of 3% or less ispreferable since uneven dyeing seldom occurs. The more preferable inertcontent is 1% or less.

The present invention related to the process for producing themultifilament will be described hereinafter. In the present invention,the multifilament is spun at a spinning speed of 3,000 m/min or more and5,000 m/min or less, drawn at a draw magnification ratio of 1,3 or moreat a draw temperature of 100 to 125° C., and subjected to heat-settingat 125 to 150° C.

Crystal orientation becomes insufficient at a spinning speed of lessthan 3,000 m/min to make work efficiency of the filament very poor dueto break of fibers at a draw temperature of 110° C. or more. A spinningspeed of exceeding 4,500 m/min makes the filament uneven to generateuneven spots by cooling, thereby causing unstable work efficiency ofspinning.

Crystal orientation is prevented from advancing at a draw temperature ofless than 110° C. break of fibers and uneven spots by drawing causes.Too high draw temperature of exceeding 125° C. causes break of fibersduring the draw step.

The tensile strength of the fiber becomes as low as less than 4.1cN/dtex causing many troubles in the processing step such as break offibers during weaving and knitting, unless the draw magnification factorexceeds 1.3. A draw magnification factor of 1.3 or more makes the fibersavailable for various processing by adjusting the elongation percentage.The draw magnification factor is preferably 1.3 to 1.8, more preferably1.5 to 1.7, considering balance between the tensile strength andelongation percentage.

A too low heat-set temperature of lower than 125° C. makes thecontraction ratio in boiling water high, and the fiber cannot be useddue to large contraction in the post-processing. A heat-set temperatureof exceeding 150° C. causes break of fibers since the temperature isclose to the melting point of the polylactic acid fiber. Therefore, asetting temperature of 135 to 150° C. is preferable consideringproductivity of the filament.

The process for producing the polylactic acid multifilament according tothe present invention will be described hereinafter.

In the process for producing the polylactic acid multifilament accordingto the present invention, the polylactic acid resin having a selectedcomposition and property above mentioned is melt-spun, drawn between theroller heaters (1) and (2), and heat-set at the roller heated (2). Theproducing process is illustrated in FIG. 1.

The conventional process is illustrated in FIG. 2. In this process, thenon-drawn fiber 10 is drawn between a roller heater (21) and cold roller(23), heat-set at a plate heater (22) and rolled up through the coldroller to obtain rolled drawn fiber 20.

The roller heater (1) is preferably heated at 100 to 125° C. fororientation and crystallization of the multifilament in the producingprocess according to the present invention.

The multifilament according to the present invention should be heat-setat the roller heater (2). Using the roller heater permits the draw pointto be fixed at just under the roller heater (1), thereby enabling thelinear density (tex) of the fine fibers from being uneven.

The irregular linear density (tex) of the fine fiber is preferablyrestricted within ±10%, more preferably within ±7% or less, relative tothe diameter of the multifilament. This range allows irregular dyeing tobe prevented with favorable dyeing.

The heat-set temperature of the roller heater (2) is preferably in therange of 125 to 150° C. considering the contraction ratio in boilingwater of the fiber obtained. The temperature is preferably 135 to 150°C. considering productivity of the filament.

EXAMPLE

The embodiments of the present invention will be described withreference to examples.

The processes for measuring and evaluating each property will bedescribed first. Measurements and evaluations other than described belowwere carried out in accordance with the processes as hitherto described.

(Birefringence)

The birefringence of the fiber was measured by a Berek compensatormethod using α-bromonaphthaline as an immersion solution.

(Thermal Stress)

A thermal stress measuring instrument TYPE KE-2S made by KaneboEngineering Co. was used.

(Fatigue after Dyeing)

A cylindrical knit sample was prepared using the multifilament, and thesample was dyed under an atmospheric pressure using a disperse dye.Fatigue of the sample after dyeing was totally evaluated in three stepsof A, B and C:

-   -   A: very good (not fatigue at all)    -   B: good    -   C: poor (fatigue is so large that the product is not applicable        as commercial products)        (Inert-Fiber with Irregular Linear Density))

Irregularity in the diameter of the multifilament obtained by ameasuring speed of 50 m/min and twist speed of 5,000 rpm was determinedin percentage using USTER-TESTER 4 made by Zelbeger-Uster Co.

(Dyeing)

A test textile was woven using the filament after drawing, and thetextile was dyed under an atmospheric pressure using a disperse dye.Dyeing of the textile was evaluated in two steps (o and x) based onirregular dyeing, dimensional stability and pilling.

-   -   o: uniform dyeing    -   x: irregular dyeing        (Polymerization of Polymer)

Polylactic acid was synthesized by a process known in the art usingL-lactide or D-lactide as a starting material and tin octylate as apolymerization catalyst. Trimellitic acid in a concentration of 0.1 mol% as a cross-link agent was added for polymerization for comparison. Thepolymer obtained was further polymerized at 135° C. in the solid phaseto reduce the amount of remaining monomers. However, no solid phasepolymerization was applied for a part of the examples as comparativeexamples.

Examples 2-1, and 2-2 and Comparative examples 2-1 to 2-5

Table 2-1 shows the results of evaluations of spinning ability and (1),(2) and service life of the nozzle when the polymers with variouscontents of Sn are spun at a spinning speed of 3,800 m/min.

With respect to Comparative Examples 2-1 to 2-3, the polymer wasdepolymerized during spinning due to particularly high content of Sn(residual catalyst). In addition, the rate of decrease of viscosityduring spinning was very high to make spinning quite difficult, and theservice life of the nozzle was as short as 1 day. Therefore, the polymerin these comparative examples are not practically applicable.

TABLE 2-1 Examples Comparative Examples No. 2- 1 2 1 2 3 4 5 Sn Content(ppm) 26 17 824 412 82 62 35 Relative 2.93 2.98 2.96 2.95 2.97 2.94 3.00Viscosity of Polymer (ηrel) Monomer Content 0.26 0.25 0.26 0.23 0.250.24 0.26 (% by weight) Mw 12.5 × 10⁴ 13.9 × 10⁴ 13.9 × 10⁴ 13.9 × 10⁴13.7 × 10⁴ 13.5 × 10⁴ 14.4 × 10⁴ Mn  6.6 × 10⁴  6.9 × 10⁴  6.8 × 10⁴ 6.7 × 10⁴  6.9 × 10⁴  6.6 × 10⁴  7.0 × 10⁴ Branched Non Non Non Non NonNon Non Structure L-isomer (mol %) 97.8 96.4 96.4 97.0 96.6 95.5 97.1Spinning 230 230 230 230 230 230 230 Temperature (° C.) Rate of Decrease5.0 3.6 73.6 64.3 52.3 17.6 12.3 of Viscosity during Spinning (%)Spinning 3800 3800 3800 3800 3800 3800 3800 speed (m/min) SpinningAbility 1 A A C C C C-B B Spinning Ability 2 ≧7 ≧7 1 1 1 3 6

While the rate of decrease of viscosity during spinning was improved to17.6% in the polymer in Comparative Example 2-4, the service life of thenozzle was only three days due to large content of Sn, which makes thepolymer practically inapplicable.

The service life of the nozzle was prolonged to six days since the rateof decrease of viscosity during spinning was improved to 12.3%. However,the service life of seven days or more could not be attained since thecontent of Sn was as high as 35 ppm. The polymers in Examples 2-1 and2-2 was excellent in spinning ability because the rate of decrease ofviscosity during spinning was as small as 5.0% due to the small contentof Sn of 50 ppm or less with sufficient service life of the nozzle.

Examples 2-3 to 2-5, and Comparative Examples 2-6 to 2-9

Table 2-2 shows the results of spinning ability and service life of thenozzle when the spinning speed was adjusted to 3,500 m/min by varyingthe content of the monomer in the polymer.

With respect to Comparative Examples 2-6 to 2-8, the polymer washeat-decomposed during spinning due to particularly high content of themonomer in the polymer. In addition, spinning was quite difficult due tolarge rate of decrease of viscosity during spinning besides the servicelife of the nozzle was as short as one day, making the polymerpractically inapplicable.

In the Comparative Example 2-9, the monomer content is still so highbesides the service life of the nozzle is only five days, thereby alsomaking the polymer practically inapplicable.

With respect to Examples 2-3 to 2-5, heat decomposition was suppressedby reducing the monomer content to 0.5% by weight or less. Consequently,the rate of decrease of viscosity during spinning was improved to 5% orless, also making spinning ability, service life of the nozzle andoccurrence of fluffs during drawing quite favorable.

TABLE 2-2 Example Comparative Example No. 2- 3 4 5 6 7 8 9 Monomer 0.460.26 0.15 10.2 5.76 3.46 0.98 Content (% by weight) Relative 2.97 2.962.56 2.96 2.89 2.92 3.02 Viscosity of Polymer (ηrel) Branched Non NonNon Non Non Non Non Structure Sn Content 19 21 16 18 19 18 17 (ppm)L-isomer 96.8 98.4 98.4 95.4 96.0 95.6 96.5 (mol %) Mw 13.8 × 10⁴ 14.0 ×10⁴ 14.4 × 10⁴ 13.9 × 10⁴ 13.7 × 10⁴ 12.5 × 10⁴ 14.4 × 10⁴ Mn  6.8 × 10⁴ 6.9 × 10⁴  7.0 × 10⁴  6.7 × 10⁴  6.9 × 10⁴  6.6 × 10⁴  7.0 × 10⁴Spinning 230 230 230 230 230 230 230 Temperature (° C.) Spinning 35003500 3500 3500 3500 3500 3500 speed (m/min) Rate of 5 2 1.5 25 20 15 10Decrease of Viscosity during Spinning (%) Spinning A A A C C C B Ability1 Spinning ≧7 ≧7 ≧7 1 1 2 5 Ability 2

Examples 2-6 to 2-7, and Comparative Examples 2-10 to 2-14

Tables 2-3 and 2-4 show productivity and properties of the multifilamentby changing the proportion of the L-isomer, the molecular weight andrelative viscosity of the polymer with or without the branched structurewith the spinning speed and draw conditions constant, wherein thecontents of Sn and monomers are adjusted to 30 ppm or less and 0.5% byweight, respectively.

While the polymers in Example 2-6 and Comparative Example 2-10 havesimilar properties with each other except presence/absence of thebranched structure, the polymer having the branched structure inComparative Example 2-10 has somewhat poor spinning ability whilegenerating fluffs during spinning. The tensile strength of the fiber wasless than 3.5 cN/dtex, which is smaller than that of the fiber having nobranched structure, and the peak temperature of thermal stress was 85°C. or less, causing fatigue of dyeing to make the fiber practicallyinapplicable.

Crystal orientation is hardly advanced during spinning and drawing inthe fiber of Comparative Example 2-14 in Table 2-4 having the proportionof the L-isomer of less than 95 mol %. The tensile strength thereof isas small as less than 3.5 cN/dtex with the contraction ratio in boilingwater of 30% or more. Therefore, the fiber is practically inapplicableas the multifilament due to poor dimensional stability in usual weaveand knit processing.

Since the fiber of Comparative Example 2-11 has a low molecular weightand relative viscosity, spinning and drawing ability becomes poor andthe tensile strength thereof is as small as less than 3.5 cN/dtex. InComparative Examples 2-12 and 2-13, on the other hand, the molecularweight and relative viscosity is so high that the spinning temperatureis forced to be elevated. Increasing the spinning temperature results inthe rate of decrease of viscosity during spinning to increase to 15% ormore to make spinning and drawing ability poor with appearance of fluffsduring drawing, thereby making the fiber to be practically inapplicable.

TABLE 2-3 Examples No. 2- 6 7 Monomer Content (% by weight) 0.27 0.27Relative Viscosity of Polymer (ηrel) 3.02 3.68 Branched Structure NonNon Sn Content (ppm) 18 17 L-isomer (mol %) 98.7 96.0 Mw 14.6 × 10^(□)19.5 × 10^(□) Mn  7.2 × 10^(□)  9.4 × 10^(□) Spinning Temperature (° C.)230 230 Rate of Decrease of Viscosity during 3 4 Spinning (%) Spinningspeed(m/min) 3500 3500 Spinning Ability 1 A A Spinning Ability 2 ≧7 ≧7Draw temperature (° C.) 110 110 Draw magnification factor 1.70 1.70 Settemperature (° C.) 145 145 Spinning Ability 3 A A Fluffs ◯ ◯Productivity of Filament A A Tensile Strength (cN/dtex) 4.43 4.38Elongation (%) 30.3 30.8 Contraction Ratio in Boiling Water (%) 9.8 14.8Birefringence Δn 0.0350 0.0367 Peak temperature of thermal stress (° C.)90 91 Fatigue after dyeing A A

TABLE 2-4 Comparative Example No. 2- 10 11 12 13 14 Monomer Content (%by 0.26 0.26 0.25 0.24 0.27 weight) Relative Viscosity of 3.04 2.58 4.024.03 3.02 Polymer (ηrel) Branched Structure Yes Non Non Yes Non SnContent (ppm) 19 18 20 18 21 L-isomer (mol %) 99.0 96.4 97.0 98.7 92.6Mw 14.8 × 10⁴ 10.2 × 10⁴ 23.8 × 10⁴ 24.0 × 10⁴ 14.5 × 10⁴ Mn  7.6 × 10⁴ 5.4 × 10⁴ 12.1 × 10⁴ 12.4 × 10⁴  7.1 × 10⁴ Spinning Temperature 230 230245 245 230 (° C.) Rate of Decrease of 6 8 15 20 3 Viscosity duringSpinning (%) Spinning speed (m/min) 3500 3500 3500 3500 3500 SpinningAbility 1 B B C C A Spinning Ability 2 4 4 5 3 ≧7 Draw temperature (°C.) 110 110 110 110 110 Draw magnification 1.70 1.70 1.70 1.70 1.70factor Set temperature (° C.) 145 145 145 145 145 Spinning Ability 3 B CC C B Fluffs X X X X X Productivity of Filament C B C C B TensileStrength 3.51 3.37 3.55 3.41 2.67 (cN/dtex) Elongation (%) 29.6 28.730.2 29.8 30.3 Contraction Ratio in 10.2 10.1 9.7 10.2 30.5 BoilingWater (%) Birefringence Δn 0.0276 0.0265 0.0289 0.0266 0.0235 Peaktemperature of 82 81 81 82 80 thermal stress (° C.) Fatigue after dyeingC B B C C

Examples 2-8 to 2-10, Comparative Examples 2-15 to 2-19

Tables 2-5 and 2-6 show the results of spinning work efficiency andproperties of the multifilament of the polylactic acid polymer having arelative viscosity of 3.09, L-isomer content of 98.2 mol % and monomercontent of 0.26% by weight without any branched structure based on theresults in Tables 2-1 to 2-4 when the spinning and drawing conditionsare changed.

While Example 2-8 and Comparative Example 2-15 show the results obtainedby changing the draw magnification factor of the fibers spun under thesame condition, the fiber with the draw magnification factor of 1.3 orless in Comparative Example 2-15 has so low tensile strength andbirefringence that the multifilament thereof is not suitable forpractical applications.

Comparative Example 2-16 shows the result obtained by reducing thespinning speed to 2,800 m/min. However, crystal orientation is soinsufficient at a reel speed of 2800 m/min that the fiber cannot endurethe draw temperature, and break of fiber often occurs to makeproductivity of the multifilament low for practical purposes.

Example 2-9 and Comparative Example 2-17 show the results obtained bychanging the draw temperature after reeling the fibers under the samecondition. Since the draw temperature in Comparative Example 2-17 islower than 100° C., break of fibers and generation of fluffs are oftenobserved due to insufficient draw temperature. The fiber obtained has solow tensile strength and birefringence that it is not practicallyapplicable.

Example 2-9 and Comparative Example 2-18 show the results obtained bychanging the set temperature after reeling the fibers under the samecondition. Since the contraction ratio in boiling water is as high as20% or more due to lower set temperature than 125° C. in ComparativeExample 2-18, the fiber is not practically applicable because thedimensional stability in post-processing such as dyeing is poor.

Comparative Example 2-19 shows the results obtained by spinning at aspeed exceeding 4,500 m/min. Although vibration of fibers, uneven fibersby cooling and break of fibers are often observed at a spinning speed of4,800 m/min to make the fiber practically inapplicable, any problems areseen with respect to spinning and drawing at the spinning speed of 4,500m/min in Example 2-10, and the multifilament obtained had good physicaland chemical properties.

TABLE 2-5 Example No. 2- 8 9 10 Spinning Temperature (° C.) 230 230 230Rate of Decrease of Viscosity during 3 3 3 Spinning (%) Spinning speed(m/min) 3200 4000 4500 Spinning Ability 1 A A A Spinning Ability 2 ≧7 ≧7≧7 Draw temperature (° C.) 105 115 120 Draw magnification factor 1.7 1.51.3 Set temperature (° C.) 145 135 150 Spinning Ability 3 A A A Fluffs ◯◯ ◯ Productivity of Filament A A A Tensile Strength (cN/dtex) 4.32 4.454.50 Elongation (%) 27.6 28.9 30.0 Contraction Ratio in Boiling Water10.2 9.8 9.7 (%) Birefringence Δn 0.0332 0.0386 0.0394 Peak temperatureof thermal 87 92 93 stress (° C.) Fatigue after dyeing A A A

TABLE 2-6 Comparative Example No. 2- 15 16 17 18 19 Spinning Temperature(° C.) 230 230 230 230 230 Rate of Decrease of Viscosity 3 3 3 3 3during Spinning (%) Spinning speed (m/min) 3200 2800 4000 4000 4800Spinning Ability 1 ◯ ◯ ◯ ◯ X Spinning Ability 2 ≧7 ≧7 ≧7 ≧7 ≧7 Drawtemperature (° C.) 105 105 90 105 120 Draw magnification factor 1.2 1.91.5 1.5 1.3 Set temperature (° C.) 150 150 150 115 150 Spinning Ability3 C C B A C Fluffs X X X ◯ X Productivity of Filament C C B B C TensileStrength (cN/dtex) 2.83 3.64 3.50 4.30 4.18 Elongation (%) 35.0 27.627.4 28.6 25.4 Contraction Ratio in Boiling 15.0 11.7 10.5 20.7 9.8Water (%) Birefringence Δn 0.0251 0.0271 0.0281 0.0310 0.0364 Peaktemperature of 78 81 79 83 90 thermal stress (° C.) Fatigue after dyeingC B B C B

Example 3-1 and 3-2, Comparative Examples 3-1 to 3-8

Each polylactic acid polymer was melted at a given temperature and spunfrom a nozzle with a nozzle diameter of 0.3 mm. The fiber was reeled ata speed of 3,000 m/min followed by drawing to prepare a multifilamentwith a size of 84 dtex/24 f, and dye affinity of the fiber wasevaluated.

Comparative Examples 3-1 and 3-2 show the results when the contents ofresidual Sn and monomers are large. Spinning ability is not so good dueto large decrease of viscosity during spinning when the contents ofresidual Sn or monomers are large. Generation of fluffs was observedduring drawing and pilling was observed during dyeing, respectively, tomake the quality of the filament poor.

The quality of the fiber in Comparative Example 3-3 was poor since thetensile strength was low and generation of fluffs was observed due tolow viscosity and molecular weight (Mw and Mn) of the polymer. Thequality of the fiber in Comparative Example 3-4 was also poor since theviscosity and molecular weight (Mw and Mn) of the polymer was so highthat the spinning temperature was forced to be elevated, thereby causinglarge decrease of viscosity during spinning, and generating fluffsduring drawing and pilling during dyeing.

While Comparative example 3-5 shows the polymer having similarproperties as the polymer in Example 1 except the presence/absence ofthe branched structure, the fiber obtained from the polymer having thebranched structure in Comparative Example 3-5 generated fluffs duringdrawing and dye affinity was poor.

In Comparative Examples 3-7 and 3-8, and in Examples 3-1 and 3-2,heat-setting after drawing was applied using a roller heater in theexamples and using a plate heater in the comparative examples for thecomparative purposes. The drawing points in the filament are not fixedin the filament heat-set using the plate heater, inert content andirregular dying are not improved by changing the set temperature, andthe filament was irregularly dyed to make the filament quality poor. Dyeaffinity was good, on the contrary, in the filament prepared by rollerheater setting without arising irregular dying.

TABLE 3-1 Comparative Example No. 3- 1 2 3 4 5 6 7 8 Sn Content (ppm) 6218 16 15 19 21 16 16 Relative Viscosity of 2.94 2.92 2.50 4.02 3.04 3.053.05 3.05 Polymer (ηrel) Monomer Content (% 0.24 1.02 0.25 0.24 0.260.27 0.24 0.24 by weight) Mw/10⁴ 13.5 14.4 10.0 23.8 14.8 14.5 14.8 14.8M□/10⁴ 6.6 7.0 5.0 12.1 7.6 7.1 7.6 7.6 Branched Structure Non Non NonNon Yes Non Non Non L-isomer (mol %) 95.5 98.2 97.6 97.0 99.0 92.6 98.698.6 Spinning Temperature 230 230 230 245 230 230 230 230 (° C.) Rate ofDecrease of 18 10 16 15 6 3 4 4 Viscosity during Spinning (%) Drawmagnification 1.7 1.7 1.7 1.7 1.7 1.7 1.7 1.7 factor roller heater (1) °C. 110 110 110 110 110 110 110 110 roller heater (2) ° C. 135 135 135135 135 135 — — plate heater ° C. — — — — — — 135 115 Tensile strength2.65 3.34 2.83 3.55 3.51 2.67 4.52 4.55 (cN/dtex) Elongation (%) 26.327.6 26.8 30.2 29.6 30.3 30.3 30.5 Contraction ratio in 11.2 10.2 10.210.3 10.2 30.5 9.6 15.0 boiling water (%) inert (U %) 1.78 1.23 1.831.82 1.54 1.56 3.80 2.50 Uneven fiber (%) ±6 ±5 ±5 ±6 ±5 ±5 ±15 ±10Fluffs X X X X X ◯ ◯ ◯ dye affinity X X X X X X X X

TABLE 3-2 Example No. 3- 1 2 Sn Content (ppm) 16 16 Relative Viscosityof Polymer (ηrel) 3.05 3.05 Monomer Content (% by weight) 0.24 0.24Mw/10⁴ 14.8 14.8 M□/10⁴ 7.6 7.6 Branched Structure Non Non L-isomer (mol%) 98.6 98.6 Spinning Temperature (° C.) 230 230 Rate of Decrease ofViscosity during Spinning (%) 4 4 Draw magnification factor 1.7 1.7roller heater (1) ° C. 110 110 roller heater (2) ° C. 135 150 plateheater ° C. — — Tensile strength (cN/dtex) 4.54 4.57 elongation (%) 28.727.6 Contraction ratio in boiling water (%) 9.6 8.0 inert (U %) 1.201.19 uneven fiber (%) ±5 ±5 Fluffs ◯ ◯ dye affinity ◯ ◯(Staple Fiber and Producing Process Thereof)

Staple fiber and producing processes thereof will be described in detailhereinafter.

Although staple fibers comprising polylactic acid compositions andproducing processes thereof have been disclosed, most of them were inlaboratory levels, and conditions for industrial production have notbeen made clear.

However, assay of the L-isomer in the polylactic acid as a startingmaterial, prescription of the degree of polymerization of the polymer,the content of monomers, catalyst and molecular structure as well asrate of thermal contraction of the staple fibers are crucial factors forpractical production and applications.

Japanese Patent Application Laid-open No. 6-212511 and 7-11515 disclosebriefly spinning and drawing processes of poly-L-lactic acid withdifferent melt flow rates (MFR), and viscosity characteristics duringmelt-spinning of aliphatic polyesters. However, since most of variousconditions required at the practical production site have not been madeclear, it is currently impossible to obtain practically applicablepolylactic acid staple fibers.

The present invention provides staple fibers of the polylactic acidcomposition capable of practical applications with good productivity byusing the polylactic acid composition having selected properties. Moreparticularly, the present invention provides the staple fibers of thepolylactic acid composition having good thermal contractioncharacteristics, an excellent tensile strength and good crimp propertiesas well as processing stability, and a process for producing the same.

Although the polylactic acid composition according to the presentinvention use L-lactic acid or D-lactic acid, or L-lactide or D-lactideas a dimer of lactic acid, or mesolactide as a starting material, it iscrucial that the composition contains 95 mol % or more, preferably 98mol % or more, of the L-isomer. Increasing the proportion of theD-isomer makes the polymer amorphous, and physical and chemicalproperties of the fiber obtained is deteriorated due to poor crystalorientation by spinning and drawing. The tensile strength isparticularly decreases and heat contraction ratio increases to make thefiber to be practically inapplicable.

The polylactic acid composition according to the present invention has arelative viscosity of 2.7 to 3.9. A sufficient tensile strength cannotbe obtained due to poor heat resistance of the polymer when the relativeviscosity is lower than this range. When the relative viscosity ishigher than this range, on the contrary, the spinning temperature isforced to be elevated to cause thermal degradation of the polymer duringspinning. Accordingly, the relative viscosity is preferably in the rangeof 2.9 to 3.6, more preferably 2.9 to 3.6, because the relativeviscosity in this range permits heat degradation during spinning to besmall.

The lower the rate of decrease of relative viscosity during spinning isdesirable, and the preferable rate is 7% or less. The polymer is seldomdecomposed and break of fibers hardly occurs during spinning when therate of decrease of the relative viscosity is less than 7%, therebyenabling good spinning ability to be attained and the tensile strengthin the drawing step to be large.

The weight average molecular weight Mw and number average molecularweight Mn of the polylactic acid composition according to the presentinvention are preferably in the ranges of 120,000 to 220,000 and 60,000to 110,000, respectively. While the molecular weight in this rangeaffords good spinning ability and sufficient tensile strength to beattained, the molecular weight out of this range causes a large decreasein the molecular weight to fail in obtaining the objective tensilestrength.

The polylactic acid composition according to the present invention has amonomer content of 0.5% by weight or less, preferably 0.3% by weight orless, and more preferably 0 or 0.2% by weight or less. The monomer asdetermined in the present invention refers to the component having amolecular weight of 1,000 or less as determined by a GPC assay.Throughput of the process extremely decreases at a monomer content ofmore than 0.5% by weight, because heat decomposition of the monomerdecreases heat resistance of the polylactic acid composition.

For reducing monomer content in the polylactic acid composition,unreacted monomers are removed by evacuating the reaction vessel atimmediately before completion of the polymerization reaction,polymerized chips are washed with an appropriate solvent, or thepolylactic acid is manufactured by solid state polymerization.

The polylactic acid composition according to the present invention isrequired to contain 30 ppm or less of Sn, preferably 0 or 20 ppm orless, in the polymer. While an Sn based catalyst is used as apolymerization catalyst of the polylactic acid composition, a content ofSn of more than 30 ppm allows spinning work efficiency to be markedlyreduced since the filtration pressure at the nozzle rapidly increases duto depolymerization during spinning.

For reducing the content of Sn, the content of Sn for polymerization isreduced or the chips obtained are washed with an appropriate solvent.

It is crucial that the polylactic acid composition according to thepresent invention has a linear polymer structure, or substantially hasno branched structure. Although a small amount of branching agent wasadded for improving melt viscosity and degree of polymerization inpolymerizing the polylactic acid composition in the conventionalproposal, it was confirmed by the inventors of the present inventionthat the branched structure of the polylactic acid composition has farmore negative effect on spinning work efficiency than the conventionalsynthetic fiber, for example a polyester fiber, has. In other words, thepolylactic acid composition containing even a trace amount of thebranched structure has poor spinning work efficiency and smaller tensilestrength as compared with the composition having no branched structure.

It is recommended not to use such agents as forming a branched structureat all in the polymer material, for example three valent or four valentalcohols and carboxylic acids. When a component having the structure asdescribed above is forced to use for some reasons, the quantity thereofshould be restricted within as small range as possible that does notaffect spinning work efficiency.

Polylactic acid to be used in the present invention preferably exhibitsa mass reduction of 5% at a temperature of 300° C. or more. Thermaldegradation in producing and processing textiles may be more preventedas TG (5%) is higher.

While commonly used resin components other than polylactic acid may beused in the polylactic acid staple fiber according to the presentinvention, biodegradable resin materials such as aliphatic polyestersare preferably used for the biodegradable staple fiber.

The staple fiber of the polylactic acid composition according to thepresent invention is manufactured by the steps of melt-spinning thepolylactic acid composition by a conventional method, drawing under acondition to be described hereinafter, mechanically crimping the spunfiber, and cutting into staples after heat-treatment.

The melt-spin temperature is preferably 215 to 250° C. Melt-extrusion iseasy at a temperature of 215° C. or more, and decomposition may beremarkably suppressed at a temperature of 250° C. or less, therebyenabling high strength staple fibers to be obtained.

The fiber after melt-spinning are cooled to ensure a desired crystalorientation, and are housed in a cans as non-drawn fibers at a speed of600 to 1200 m/min. A speed less than 600 m/min makes reeling difficultdue to insufficient tension of the fiber, while a speed exceeding 1,200m/min make it difficult to house in a cans due to high speed spinning.The speed is preferably 900 to 1,100 m/min.

The non-drawn fiber is drawn by one or two steps at a draw temperatureof 50 to 98° C. and draw magnification factor of 3.0 to 5.0, preferably3.5 to 4.5. A draw magnification factor of less than 3.0 is notpractical since the elongation is too large, while the elongationreduces and mechanical load increases and productivity of drawingreduces when the draw magnification factor exceeds 5.0.

While the draw magnification factor is different depending on thespinning speed and required performance of the staple fiber, it isadjusted so that a fiber having a tensile strength of 2.6 cN/dtex ormore and an elongation of 80% or less is obtained.

The heat treatment may be applied before or after the crimp processing.The heat treatment temperature is adjusted to 110 to 150° C., preferably120 to 140° C., for adjusting the heat contraction ratio at 120° C.within 5.0%.

The thermal contraction ratio of the fiber of the polylactic acidcomposition staple fiber according to the present invention at 120° C.is preferably 5.0% or less, more preferably 3.0% or less. The fiberbecomes suitable for practical applications when the thermal contractionratio at 120° C. is 5.0% or less, since contraction by heat treatment ofthe fabric and dyeing hardly occurs and feeling of the fabric issuppressed from changing when the staple fiber is processed into atextile product of the spun fiber. The fiber may be used for the shortstaple nonwoven fabric through a dry or wet process, irrespective ofthermosetting temperatures.

The staple fiber of the polylactic acid composition according to thepresent invention preferably has a tensile strength of 2.6 cN/dtex ormore, more preferably 3.5 cN/dtex or more. The tensile strength of 2.6cN/dtex or more is preferable because no troubles are encountered in theprocessing step and in practical uses with a sufficient strength of thefinal product.

Practically preferable elongation is 80% or less, more preferably 60% orless.

The number of crimps of the fiber of the polylactic acid compositionaccording to the present invention is preferably 4 to 18 crimps/25 mm,more preferably 6 to 15 crimps/25 mm. Non-dispersed part of the fiberhardly appears when the crimp number more than 4 crimps/25 mm, whilegeneration of neps is suppressed when the crimp number is less than 18crimps/25 mm.

When the fiber is endowed with crimps by a stuffing box method, towsbefore entering the crimper is pre-heated at 40 to 100° C., and the towsare passed through the crimper with a nip pressure of 0.2 to 0.4 MPa anda press pressure of 0.03 to 0.10 MPa to attain the crimp number ashitherto described.

The fiber is heat-treated at 120 to 140° C. for setting the objectivethermal contraction ratio to 5.0% or less.

Oil may be coated before or after drying, and the fiber is cut with acutter to form staple fibers. The staple fiber thus obtained isexcellent in productivity while having good thermal contractionproperties, tensile strength and crimp characteristics in addition tostability in processing.

The linear density (tex) of a single fiber is usually in the range of0.6 to 22 dtex.

The staple fiber according to the present invention is processed as awoven or knit product by a conventional weave and knit process, or as ashort staple nonwoven fabric by a dry or wet process.

EXAMPLES

The present invention will be described in detail with reference toexamples.

The analysis processes of the polymer properties and measuring processesof the textile properties will be described first. The properties notdescribed hereinafter have been measured and evaluated by the foregoingprocesses.

(Measurement of Thermal Contraction—Dry Method)

An initial load of 1.8 μN/dtex was given to a sample with a length of 25mm to measure the initial length. Then, the length of the sample aftertreating with a hot-air dryer at 120° C. for 15 minutes (the samplelength after contraction) was measured to determine the thermalcontraction ratio by the equation below:Thermal contraction ratio (%)=[(Initial sample length−Sample lengthafter contraction)/Initial sample length]×100

Example 4-1

Polylactic acid was synthesized by a conventional method using tinoctylate as a polymerization catalyst with a starting material ratio of98.7 mol % of L-lactide and 1.3 mol % of D-lactide. The polymer obtainedhad a relative viscosity of 3.02, weight average molecular weight Mw of146,000 and number average molecular weight Mn of 72,000 with a monomercontent of 0.27% by weight, Sn content of 18 ppm and heat stabilitytemperature TG (5%) of 318° C.

The polymer was melt-spun at an extrusion mass rate of 715 g/min andspinning speed of 1,050 m/min at a spinning temperature of 230° C. froma spinning nozzle with a diameter of 0.27 mm and number of spinningholes of 1420. The non-drawn fiber was pulled into a cans after coolingby in an annular air stream. The rate of decrease of viscosity duringspinning was 3% and the incidence of break of fibers was 0.73 times/ton.

After pre-heating the non-drawn fiber at 40° C., it was drawn at a drawmagnification factor of 3.96 at 85° C. followed by heat-treating at 110°C. under a tension. Rill times of on the roller during drawing was afavorable value of 0.24 times/ton.

The drawn tows were crimped by introducing into a crimper (a nippressure of 0.25 MPa, stuffing pressure of 0.05 MPa) while heating at85° C. with steam. Then, the crimped tows were dried and heat-treated at130° C. with a hot-air dryer. After coating with an oil, the tows werecut in to a length of 38 mm to obtain staple fibers with a liner densityof 1.1 dtex. The staple fiber obtained had a thermal contraction ratioat 120° C. of 2.7%, a tensile strength of 4.0 cN/dtex or more, anelongation of 45.4%, and a number of crimps of 10.6 crimps/25 mm.Spinning ability of this staple fiber was good with satisfactory thermalcharacteristics and tensile strength of spun fiber. This staple fiber ismainly used for mix spinning with cotton.

Comparative Example 4-1

Polylactic acid was synthesized by a conventional method using tinoctylate as a polymerization catalyst with a mixing ratio of thestarting materials of 99.0 mol % of L-lactide and 1.0 mol % of D-lactidetogether with 0.1 mol % of trimellitic acid as a cross-link agent.

The polymer obtained had a relative viscosity of 3.04, a weight averagemolecular weight Mw of 148,000, a number average molecular weight Mn of76,000, a monomer content of 0.26% by weight and an Sn content of 19ppm. The heat stability temperature TG (5%) was 315° C.

A non-drawn fiber was reeled under the same condition as in Example 4-1.Although the rate of decrease of viscosity during spinning was 6%,spinning ability was not good with an incidence of break of fibers of2.43 times/ton.

The non-drawn fiber was drawn under the same condition as in Example 1,whereby rill on the roller during drawing was as poor as 1.21 times/ton.

Example 4-2

Polylactic acid was synthesized by a conventional method using tinoctylate as a polymerization catalyst with starting material ratios of97.8 mol % of L-lactide and 2.2 mol % of D-lactide. The polymer obtainedhad a relative viscosity of 2.93, weight average molecular weight Mw of125,000, number average molecular weight Mn of 66,000, monomer contentof 0.26% by weight and Sn content of 26 ppm. The heat stabilitytemperature TG (5%) was 317° C.

The polymer was melt-spun at a spinning temperature of 230° C., spinningspeed of 950 m/min with an extrusion mass rate of 800 g/min from aspinning nozzle with a diameter of 0.40 mm and number of spinning holesof 820. The non-drawn fiber was pulled in cans after cooling in anannular air stream. The rate of decrease of viscosity during spinningwas 5%, and incidence of break of fibers was 0.22 times/ton.

After preheating the non-drawn fiber at 40° C., the non-drawn fiber wasdrawn at a draw magnification factor of 3.74 at 82° C. Reeling on theroller showed a favorable level of 0.0 times/ton.

The drawn tows were crimped by introducing into a crimper (nip pressureof 0.27 MPa and stuffing pressure of 0.06 MPa) while heating them withsteam at 85° C.

The crimped tows were dried and heat treated at 135° C. with a hot-airdryer and, after coating with an oil, were cut into a length of 51 mmwith a bias length of 76 mm to obtain staple fibers with a lineardensity of 3.3 dtex. The staple fiber obtained had a thermal contractionratio at 120° C. of 1.7%, tensile strength of 3.0 cN/dtex and elongationof 58.4% with a number of crimps of 10.9 crimps/25 mm.

The staple fiber was spun by mixing with wool. The spun fiber hadsatisfactory thermal characteristics and tensile strength, and thedyeing temperature was comparable to polyesters.

The staple fibers may be carded to use as a material of a nonwovenfabric after needle punch and heat treatment.

Example 4-3

Polylactic acid was synthesized in a starting material composition of96.8 mol % of L-lactide and 3.2 mol % of D-lactide by a conventionalmethod using tin octylate as a polymerization catalyst.

The polymer obtained had a relative viscosity of 2.96, weight averagemolecular weight Mw of 138,000, number average molecular weight Mn of80,000, monomer content of 0.47% by weight and Sn content of 19 ppm witha heat stability temperature TG (5%) of 302° C.

The polymer was melt-spun at a spinning temperature of 228° C. andspinning speed of 1,000 m/min with an extrusion mass rate of 800 g/minfrom a spinning nozzle having 320 holes in the shape of double C with aslit width of 0.15 mm. The spun fiber was cooled by blowing an annularair stream, and the non-drawn fiber was pulled in a cans. The rate ofdecrease of viscosity during spinning was 5%, and incidence of break offibers was 0.0 times/ton.

After pre-heating the non-drawn fiber at 40° C., it was drawn at a drawmagnification factor of 4.07 at 82° C. Reeling on the roller duringdrawing was a favorable level of 0.0 times/ton. The drawn tow wascrimped by introducing into a crimper (nip pressure 0.22 MPa andstuffing pressure 0.05 MPa) by heating at 85° C. with steam.

The crimped tow was dried and heat-treated at 130° C. with a hot-airdryer. After coating with an oil, the tow was cut into a length of 51 mmto obtain a staple fiber with a linear density of 7.6 dtex.

The staple fiber obtained had a thermal contraction ratio at 120° C. of3.5%, tensile strength of 3.4 cN/dtex or more, elongation of 48.2% andnumber of crimps of 8.2 crimps/25 mm.

The staple fiber smoothly passed through the card, and characteristicsof the nonwoven fabric after needle punch and heat-treatment weresatisfactory.

(Monofilament and Producing Process Thereof)

The invention with respect to the monofilament and producing processthereof will be described hereinafter.

Although the monofilament comprising the polylactic acid composition andproducing process thereof have been disclosed, most of them are in alaboratory level, and conditions for industrial production have not beenmade clear.

However, studies of the composition of polylactic acid as a startingmaterial, prescription of the degree of polymerization, monomer content,catalyst and molecular structure as well as thermal contractioncharacteristics of the monofilament will be crucial factors forpractical production and applications in the textiles, for particularlymonofilament comprising the polylactic acid composition.

While Japanese Patent Application Laid-open No. 7-90715 discloses thepolymer viscosity of aliphatic polyesters during spinning and processesfor modifying the polymer, conditions required in the practicalproduction sites as described above have not been made clear. Therefore,it has been currently impossible to obtain practically applicablepolylactic acid monofilament.

The present invention provides a practically applicable monofilament ofthe polylactic acid composition with good productivity by using thepolylactic acid composition having selected properties. Moreparticularly, the present invention provides monofilaments of thepolylactic acid composition having good thermal contractioncharacteristics and tensile strength capable of stabile processing, anda process for producing the same.

While the polylactic acid composition according to the present inventionuses L-lactic acid or D-lactic acid, or L-lactide or D-lactide as adimer of lactic acid, or mesolactide as a starting material, it iscrucial that the proportion of the L-isomer is 95 mol % or more, becausean increase of the proportion of the D-isomer brings about an amorphousstructure to inhibit crystal orientation during spinning and drawingfrom advancing, thereby making the properties of the textile obtained tobe poor. In particular, the tensile strength is remarkably reduced whileincreasing thermal contraction ratio to make the product practicallyinapplicable.

The polylactic acid composition to be used in the monofilament accordingto the present invention has a relative viscosity (ηrel) of 2.7 to 4.5.Heat resistance of the polymer becomes poor when the relative viscosityis lower than this range to fail in obtaining a sufficient tensilestrength, while the relative viscosity of higher than this range forcesthe spinning temperature to be elevated to cause heat degradation duringspinning.

The range of the relative viscosity of 2.7 or more and 3.9 or less ispreferable since heat degradation can be suppressed, and more preferablerange is 3.1 to 3.7. However, heat degradation may be suppressed evenwhen the relative viscosity exceeds 3.9 by adjusting the content of theL-isomer to 97% or more.

The lower the rate of decrease of the relative viscosity in spinning isfavorable, and a rate of 7% or less is preferable. When the rate ofdecrease of the relative viscosity is less than 7%, the polymer isseldom decomposed during spinning and break of fibers during spinninghardly occurs to enable the tensile strength to be large in the drawstep with good spinning ability.

The polylactic acid composition according to the present invention has apreferable weight average molecular weight Mw of 120,000 to 220,000,more preferably 150,000 to 200,000, and a preferable number averagemolecular weight Mn of 60,000 to 110,000, more preferably 80,000 to100,000. While a molecular weight within this range permits goodspinning ability and sufficient tensile strength to be obtained, a largedecrease of the molecular weight causes to make it impossible to obtaina required tensile strength when the molecular weight is out of thisrange.

The polylactic acid composition according to the present invention has amonomer content of 0.5% by weight or less, preferably 0.3% by weight orless and more preferably 0 or 0.2% by weight or less. The monomer asdetermined in the present invention is referred to as the monomercomponent having a molecular weight of 1,000 or less as determined by aGPC assay. The monomer content of exceeding 0.5% by weight markedlydecreases work efficiency of the polymer, because the monomer componentis decomposed by heat to decrease heat resistance of the polylactic acidcomposition.

For reducing the content of the monomer in the polylactic acidcomposition, the unreacted monomers are removed by evacuating thereaction vessel at immediately before completion of the polymerizationreaction, the polymerized chips are washed with an appropriate solvent,or the polylactic acid is polymerized by solid state polymerization.

It is essential that the polylactic acid composition according to thepresent invention contains 30 ppm or less, preferably 0 or 20 ppm orless, of Sn in the polymer. While the Sn based catalyst is used as thepolymerization catalyst of the polylactic acid composition, a content ofSn of exceeding 30 ppm allows the polymer to be depolymerized duringspinning to rapidly increase filtration pressure of the spinning nozzle,thereby remarkably reducing work efficiency of spinning.

For reducing the content of Sn the amount of Sn for polymerization maybe reduced, or the polymer may be washed with an appropriate solvent.

It is essential that the polylactic acid composition according to thepresent invention has a linear polymer structure, or substantiallycontains no branched structure. A small amount of branching agent havebeen added for polymerization of the polylactic acid composition for thepurpose of improving the melt viscosity and degree of polymerization.However, it was confirmed by the inventors of the present invention thatthe branched structure of the polylactic acid composition far morenegatively affects spinning work efficiency as compared withconventional monofilaments, for example polyester monofilaments. Inother words, the polylactic acid composition containing even a smallamount of the branched structure is poor in spinning work efficiencybesides having a lower tensile strength than the structure without anybranched structure.

For excluding the branched structure, it is recommended to avoid use ofagents that arise the branched structure, for example three valent orfour valent alcohols and carboxylic acids, in the polymer material.However, when a component having such structure is forced to use forsome reasons, the amount should be restricted within a minimum essentialrange that does not affect work efficiency of spinning.

The polylactic acid to be used in the present invention is preferablyhas a mass reduction of 5% at a temperature of 300° C. or more, or has aheat stability temperature of TG (5%) of 300° C. or more. Thermaldegradation in producing and processing textiles may be more preventedas TG is higher.

Although common resins other than polylactic acid may be used asstarting materials in the polylactic acid monofilament according to thepresent invention, the material is preferably a biodegradable resin suchas an aliphatic polyester for manufacturing a biodegradablemonofilament.

The monofilament of the polylactic acid composition according to thepresent invention is manufactured by melt-spinning the polymer by aconventional method at 220 to 250° C. followed by cooling with water,and heat-treating after heat-drawing under the following conditions.

The melt-spinning temperature is preferably 220 to 250° C., becausemelt-extrusion becomes easy at a temperature of 220° C. or more, anddecomposition is extremely suppressed at a temperature of 250° C. orless, thereby enabling a monofilament having a high tensile strength tobe easily obtained.

The melt-spun filament is drawn at a prescribed temperature and drawmagnification factor while cooling with water in order to facilitate agiven crystal orientation, and the filament is reeled on a bobbin. Thenon-drawn filament is drawn by one or two steps or more in hot water at70 to 100° C., preferably at 85 to 98° C.

The draw magnification factor is 6.0 or more, preferably 8.0 or more.The factor is different depending on the required performance of thefilament, and is determined so that a filament having a tensile strengthof 3.5 cN/dtex or more and elongation of 40.0% or less is obtained. Theheat-treatment temperature is adjusted in the range of 100 to 150° C.,preferably 120 to 140° C., for restricting the contraction ratio inboiling water to 10.0% or less.

The contraction ratio in boiling water of the monofilament of thepolylactic acid composition according to the present invention ispreferably 10.0% or less, more preferably 8.0% or less.

The filament is favorable for practical uses since the filament ishardly contracted by heat-treatment without causing no changes in thefeeling when the contraction ratio in boiling water is 10.0% or less.There will be also no problem of making the use of the textileimpossible depending on the heat-setting temperature.

The monofilament of the polylactic acid composition according to thepresent invention preferably has a tensile strength of 3.5 cN/dtex ormore, more preferably 4.4 cN/dtex or more.

No troubles will be encountered in the processing steps when the tensilestrength is 3.5 cN/dtex or more with a sufficient strength of the finalproduct to exclude troubles in practical applications.

The elongation is preferably 40.0% or less, more preferably 35.0% orless, from the practical point of view.

The birefringence Δn after drawing is preferably 0.0250 or more, morepreferably 0.0330 or more. Crystal orientation sufficiently advances andcontraction ratio in boiling water is properly suppressed when thefilament has a birefringence Δn of 0.0250 or more.

The monofilament obtained as described above is excellent inproductivity while having practically applicable thermal contractionratio and tensile strength as well as stability in processing.

The monofilament usually has a linear density of 220 to 1,100 dtex.

The monofilament according to the present invention can be used as wovenand knit fabrics manufactured by the process known in the art.

EXAMPLES

The present invention will be described hereinafter in detail. Eachmeasuring process is as hitherto described.

Example 5-1

Polylactic acid was synthesized by the conventional method using tinoctylate as a polymerization catalyst with a starting material ratio of96.0 mol % of L-lactide and 4.0 mol % of D-lactide.

The polymer obtained had a relative viscosity of 3.7, weight averagemolecular weight Mw of 195,000, number average molecular weight Mn of94,000, monomer content of 0.27% or less by weight and Sn content of 17ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted at 220° C. in a single screw extruder, and wasextruded from a nozzle having 18 spinning holes with a diameter of 1.2mm. After allowing the filament to pass through a cooling water bath, itwas subjected to a first step drawing at a draw magnification factor of5.5 in hot water at 94° C., and to a second step drawing at a drawmagnification factor of 1.2 in hot water at 98° C., followed byheat-setting in a hot air stream at 130° C. to manufacture amonofilament with a linear density of 560 dtex.

The monofilament obtained had a contraction ratio in boiling water of9.3%, tensile strength of 4.4 cN/dtex, elongation of 36%, andbirefringence Δn of 0.0325. The rate of decrease of viscosity duringspinning was 4%, suggesting small amount of decomposition of the polymerduring spinning to result in substantially no break of fibers.

The contraction ratio in boiling water of 10.0% or less allows the wovenand knit fabric to hardly contract by heat-treatment without any changesin the feeling, thus making the product to be practically applicable. Notroubles of making the fabric unusable by the heat-setting temperaturewas encountered. The tensile strength of 3.5 cN/dtex or more preventstroubles in the processing steps from occurring, and allows the strengthof the final product to be sufficient without generating practicalproblems. The elongation of 40.0% or less is suitable for practicalapplications. The birefringence of 0.0320 or more indicate well advancedcrystal orientation and adequately suppressed contraction ratio inboiling water.

Comparative Example 5-1

Polylactic acid was synthesized by the conventional method usingL-lactide and D-lactide as starting materials and tin octylate as apolymerization catalyst, and by adding 0.1 mol % of trimellitic acid asa cross-linking agent.

The polymer obtained contained 95.5 mol % of the L-isomer and had arelative viscosity of 3.7, weight average molecular weight Mw of185,000, number average molecular weight Mn of 92,000, monomer contentof 0.8% by weight and Sn content of 16 ppm with a thermal stabilitytemperature (5%) of 320° C.

The polymer was melted at 220° C. in a single screw extruder andextruded from a nozzle having 18 spinning holes with a diameter of 1.2mm.

The filament was passed through a water cooling bath, subjected to afirst step drawing with a draw magnification factor of 5.5 in hot waterat 94° C. and second step drawing with a draw magnification factor of1.2 in hot water at 98° C., and heat set at 130° C. in a hot air streamto manufacture a monofilament with a linear density of 560 dtex.However, this filament was poor in spinning ability with high incidenceof break of fibers due to large proportion of cross-linked polylacticacid.

Example 5-2

Polylactic acid was synthesized by a conventional method with a startingmaterial ratio of 95.7 mol % of L-lactide and 4.3 mol % of D-lactideusing tin octylate as a polymerization catalyst.

The polymer obtained had a relative viscosity of 3.3, weight averagemolecular weight Mw of 174,000, number average molecular weight Mn of91,000, monomer content of 0.20% by weight or less and Sn content of 16ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted at 220° C. in a single screw extruder, andextruded from a nozzle having 18 spinning holes with a diameter of 1.2mm. The filament was passed through a water cooling bath, and subjectedto the first step drawing at a draw magnification factor of 6.0 in hotwater at 94° C. and second step drawing at a draw magnification factorof 1.5 in hot water at 98° C. The drawn filament was heat-set at 130° C.in a hot air stream to manufacture a monofilament with a linear densityof 560 dtex.

The monofilament obtained had a contraction ratio in boiling water of6.7%, tensile strength of 5.1 cN/dtex, elongation of 33.0% andbirefringence Δn of 0.0350. The rate of decrease of viscosity duringspinning of 4% suggests a small amount decomposition of the polymerduring spinning with substantially no break of fibers.

The contraction ratio in boiling water of 10.0% or less affordspractically favorable woven and knit products due to seldom contractionduring heat-treatment with no changes in feeling. Troubles such that theproduct becomes unusable by heat-setting temperature could be alsoavoided.

The tensile strength of 3.5 cN/dtex or more hardly arises troubles inthe processing steps with sufficient strength in the final productsavoiding any troubles in practical applications. The elongation of 40.0%or less was practically favorable.

The birefringence of 0.0320 or more indicates sufficiently advancedcrystal orientation to adequately suppress the contraction ratio inboiling water.

Example 5-3

Polylactic acid was synthesized by the conventional method using tinoctylate as a polymerization catalyst with a starting material ratio of98.9 mol % of L-lactide and 1.1 mol % of D-lactide.

The polymer obtained had a relative viscosity of 4.5, weight or lessaverage molecular weight of 230,000, number average molecular weight of116,000, monomer content of 0.2% by weight or less and Sn content of 16ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted at 228° C. in a single screw extruder, andextruded from a nozzle having 18 spinning holes with a diameter of 1.2mm. The filament was passed through a water cooling bath, and subjectedto the first step drawing with a draw magnification factor of 6.0 in hotwater at 98° C. and the second step drawing with a draw magnificationfactor of 1.85 in hot water at 98° C. with a total draw magnificationfactor of 11.1. The filament was heat-set in a hot air stream at 130° C.to manufacture a monofilament with a linear density of 560 dtex.

The monofilament obtained had a contraction ratio in boiling water of4.2%, contraction ratio after hot air treatment at 100° C. of 3.1%,tensile strength of 5.15 cN/dtex and elongation of 28.0%. The rate ofdecrease of viscosity during spinning of 4% suggests small amount ofdecomposition of the polymer during spinning to substantially arise nobreak of fibers.

The contraction ratio in boiling water of 6.0% or less and contractionratio after hot air treatment at 100° C. of 4.0% afford woven and knitproducts that scarcely arise contraction during heat-treatment. Theproduct substantially shows no changes of feeling that makes the productpractically favorable.

The tensile strength of 4.85 cN/dtex or more can prevent troubles in theprocessing steps with sufficient strength of the final product withoutany practical problems. The elongation of 30.0% or less was practicallyfavorable.

(Flat Yarn and Producing Process Thereof)

The flat yarn and producing process thereof according to the presentinvention will be described hereinafter.

In textile products from the polylactic acid composition, in particularthe flat yarn among them, the composition of polylactic acid as astarting material, prescription of the degree of polymerization of thepolymer, the monomer content, catalyst and molecular structure as wellas thermal contraction characteristics of the flat yarn are crucialfactors for practical producing and uses.

For example, Japanese Patent No. 2733184 discloses a flat yarnmanufactured by melt extrusion molding of an aliphatic polyestercomprising glycolic acid and polybasic acid as constituents. However,only the prior art is described with respect to lactic acid, and nodetailed explanation is made in the patent. Conditions required atpractical production sites have not been made clear. Therefore, it iscurrently impossible to obtain practically applicable polylactic acidflat yarns.

The present invention provides a practically applicable polylactic acidflat yarn with high productivity by using a polylactic acid compositionhaving selected properties. More particularly, the present inventionprovides a polylactic acid flat yarn having good thermal contractioncharacteristics and high tensile strength as well as stability inprocessing and producing process thereof.

While the starting material of the polylactic acid composition accordingto the present invention comprises L-lactic acid or D-lactic acid, orL-lactide or D-lactide as a dimer of lactic acid, or mesolactide, it iscrucial that the proportion of the L-isomer is 95 mol % or more. This isbecause increased proportion of the D-isomer results in an amorphousstructure, which prevent crystal orientation by drawing from advancingto make the properties of the textile obtained poor. The tensilestrength particularly decreases while increasing the thermal contractionratio to make practical applications of the textile impossible.

The polylactic acid composition according to the present invention has arelative viscosity (ηrel) of 2.7 to 4.5. The melt-extrusion temperatureshould be elevated when the viscosity exceeds the upper limit toconsequently increase thermal degradation while, when the viscosity isbelow the lower limit, heat resistance of the polymer becomes too poorto obtain a sufficient tensile strength. Accordingly, the preferablerange of the relative viscosity is 3.3 to 4.3.

The lower the rate of decrease of viscosity during melt extrusion isfavorable, and preferable rate is 7% or less. The polymer is notsubstantially decomposed by melt-extrusion when the rate of decrease ofviscosity during melt extrusion is 7% or less to exclude irregular filmsfrom being formed by melt-extrusion. Accordingly, a film having a hightensile strength during drawing may be formed with good film formingability.

The polylactic acid composition according to the present inventionpreferably has a weight average molecular weight Mw of 125,000 to230,000, more preferably 174,000 to 215,000, and number averagemolecular weight Mn of 73,000 to 116,000, more preferably 91,000 to107,000. The molecular weight in this range permits good film formingability and high tensile strength to be obtained.

The polylactic acid composition according to the present inventioncontains 0.5% by weight or less, preferably 0.3% by weight or less, andmore preferably 0 or 0.2% by weight or less of monomers. The monomer asdetermined in the present invention refers to as a monomer componenthaving a molecular weight of 1000 or less as determined by a GPC assay.The monomer content of 0.5% by weight or less is preferable forattaining high work efficiency, because heat resistance of thepolylactic acid composition becomes more excellent as the content of theheat-decomposed monomer component is smaller.

For reducing the monomer content in the polylactic acid composition,unreacted monomers may be removed by evacuating the reaction vesselimmediately before completing the polymerization reaction, polymerizedchips may be washed with an appropriate solvent, or polylactic acid isprepared by solid phase polymerization.

The content of Sn in the polylactic acid composition according to thepresent invention is required to be 30 ppm or less, preferably 0 or 20ppm or less. While the Sn based catalyst is used as a polymerizationcatalyst of the polylactic acid composition, a content of 30 ppm or lesspermits filtration pressure at the nozzle to be suppressed fromincreasing due to small amount of depolymerization during melt-extrusionto make the polymer excellent in melt-extrusion ability.

For reducing the content of Sn, the proportion of Sn used inpolymerization is reduced, or the chips are washed with an appropriatesolvent.

It is essential that the polylactic acid composition according to thepresent invention has a linear polymer structure, or substantially hasno branched structure. It has been proposed to add a small amount of abranching agent for preparing the polylactic acid composition in orderto improve the melt viscosity and degree of polymerization. However, itwas confirmed by the inventors of the present invention that thebranched structure of the polylactic acid composition far morenegatively affects film forming ability as compared with conventionalflat yarns, for example polyester flat yarns. In other words, it is aproblem that work efficiency for forming the film becomes poor in thepolylactic acid composition containing even a small quantity of branchedstructures, and tensile strength of the film is lower as compared withthe film having no branched structures.

For excluding the branched structure, it is recommended to avoid use ofagents that arise the branched structure, for example three valent orfour valent alcohols and carboxylic acids, in the polymer material.However, when a component having such structure is forced to use forsome reasons, the amount should be restricted within a minimum essentialrange that does not affect the film forming ability.

Polylactic acid to be used in the present invention preferably has atemperature for reducing the polymer mass by 5%, or TG (5%), of 300° C.or more. The higher TG (5%) is, the more heat degradation in producingand processing the flat yarn may be prevented.

Although common resins other than polylactic acid may be used asstarting materials in the polylactic acid flat yarn according to thepresent invention, the material is preferably a biodegradable resin suchas an aliphatic polyester for manufacturing a biodegradable flat yarn.

While the flat yarn of the polylactic acid composition according to thepresent invention may be manufactured by a process known in the artusing the polymer of the polylactic acid composition, one example of theproducing process comprises solidifying by cooling after melt-extrusion,and hot-drawing under the conditions to be described below followed byheat-treatment.

The melt-extrusion temperature is preferably in the range of 180 to 250°C. A melt-extrusion temperature of 180° C. or more makes melt-extrusioneasy, while a temperature of 250° C. or less extremely preventdecomposition, thereby enabling a flat yarn having a high tensilestrength to be easily obtained.

The melt-extruded film is cooled to attain a desired crystalorientation, and drawn at a prescribed temperature and drawmagnification factor followed by reeling on a bobbin afterheat-treatment. The film is slit into ribbons, which are drawn by one ortwo steps at 80 to 130° C., preferably at 100 to 120° C.

The draw magnification factor is 4.0 or more, preferably 5.0 or more.Although the factor differs depending on the required performance of theobjective flat yarn, it is determined so that a flat yarn having atensile strength of 2.6 cN/dtex or more and elongation of 40.0% or lessis obtained.

The flat yarn is preferably heat-treated at 100 to 150° C., morepreferably at 110 to 140° C., for adjusting the contraction ratio afterheat-treatment at 80° C. for 10 minutes to 5.0% or less.

The flat yarn of the polylactic acid composition according to thepresent invention preferably has preferably a contraction ratio of 5.0%or less, more preferably 3.0% or less, after heat-treating the flat yarnat 80° C. for 10 minutes. The contraction ratio of 5.0% or less afterheat-treating the flat yarn at 80° C. for 10 minutes allows contractionby heat-treatment to be hardly occurs when the yarn is processed intowoven and knit fabrics without any changes of feeling. Therefore, thefabric is favorable for use by excluding the problems that the fabricbecomes unusable by heat-setting temperature.

The flat yarn of the polylactic acid composition according to thepresent invention preferably has a tensile strength of 2.6 cN/dtex ormore, more preferably a tensile strength of 3.0 cN/dtex or more. Atensile strength of 2.6 cN/dtex or more seldom arises troubles in theprocessing steps besides having a sufficient strength in the finalproduct by excluding practical problems.

The elongation is preferably 40.0% or less, more preferably 35.0% orless, from the practical point of view.

The flat yarn thus obtained is excellent in productivity, and has goodthermal contraction characteristics and tensile strength suitable forpractical uses as well as stability in processing.

The linear density of the flat yarn is usually in the range of 330 to1100 dtex when the yarn has a width of 3 to 6 mm, and 560 to 3,300 dtexwhen the yarn has a width of 6 to 12 mm.

The flat yarn according to the present invention may be processed in towoven and knit fabrics for use by the process known in the art.

EXAMPLES

The present invention will be described hereinafter with reference toexamples, measurements of physical and chemical properties are asfollows. The properties not described below was measured by the processas hitherto described.

(Rate of Decrease of Viscosity During Melt-Extrusion)

The relative viscosity (ηrel) of the film shaped sample extruded out ofthe die was measured to determine the rate of decrease of viscosity bythe following equation. The residence time of the molten polymer wasabout 10 minutes in this example.Rate of decrease of viscosity during melt-extrusion (%)=[(relativeviscosity of polymer−relative viscosity of film)/relative viscosity ofpolymer]×100

Example 6-1

Polylactic acid was synthesized by a conventional process using tinoctylate as a polymerization catalyst with a starting material ratio of96.0 mol % of L-lactide and 4.0 mol % of D-lactide.

The polymer obtained had a relative viscosity of 3.7, weight averagemolecular weight Mw of 195,000, number average molecular weight Mn of94,000, monomer content of 0.27% by weight or less and Sn content of 17ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted in a single screw extruder at 190° C., andmelt-extruded from a circular die molding apparatus with a diameter of30 cm and a lip gap of 1.0 mm, followed by solidifying by cooling toform a raw sheet. The raw sheet was slit into 6 mm wide strips, whichwere drawn on a hot plate followed by anneal drawing with a hot airstream. The first step drawing was performed on a hot plate at atemperature of 115° C. with a draw magnification factor of 5.0, and thesecond step drawing was performed on a hot plate at a temperature of120° C. with a draw magnification factor of 1.2, followed byheat-setting at 130° C. in a hot air stream with an annealing ratio of5%, thereby obtaining a flat yarn with an width of 3 mm and lineardensity of 560 dtex.

The flat yarn obtained had a contraction ratio of 3.9%, tensile strengthof 2.9 cN/dtex and elongation of 33.0%. The rate of decrease ofviscosity during melt-extrusion of 4% suggests small amount ofdecomposition of the polymer during melt-extrusion to substantiallyarise no troubles in forming the raw sheet. The contraction ratio of5.0% or less allows contraction by heat-treatment to be hardly generatedwhen the flat yarn is processed into woven and knit fabrics with nochanges in feeling, making the fabrics practically favorable. Problemsthat the textile becomes unusable by the heat-setting temperature werenever observed. Since the tensile strength is 2.6 cN/dtex or more, notroubles were encountered in the processing steps to ensure sufficientstrength of the final product to exclude practical problems. Theelongation of 40.0% or less was practically favorable.

Comparative Example 6-1

Polylactic acid was synthesized by the conventional method using tinoctylate as a polymerization catalyst and L-lactide and D-lactide asstarting materials, and by adding 0.1 mol % of trimellitic acid as across-linking agent. The polymer obtained contained the 95.5 mol % ofL-isomer and had a relative viscosity of 3.7, weight average molecularweight Mw of 185,000, number average molecular weight Mn of 92,000,monomer content of 0.18% by weight or less and Sn content of 16 ppm witha heat stability temperature (5%) of 320° C.

The polymer was melted in a single screw extruder at 190° C., andmelt-extruded from a circular die extruder with a diameter of 30 cmhaving a lip gap of 1.0 mm, followed by solidifying by cooling to form araw sheet. Since the sheet contains cross-linked polylactic acid, manytroubles were seen in forming the raw sheet with poor melt-extrusionability. The raw sheet was slit into 6 mm wide stripes, which were drawnon a hot plate followed by anneal drawing with a hot air stream. Thefirst step drawing was performed on a hot plate at a temperature of 118°C. with a draw magnification factor of 5.0, and the second step drawingwas performed on a hot plate at a temperature of 120° C. with a drawmagnification factor of 1.2, followed by heat-setting at 125° C. in ahot air stream with an annealing ratio of 5%, thereby obtaining a flatyarn with an width of 3 mm and linear density of 560 dtex. Troublesduring drawing the flat yarn was often seen due to the presence ofcross-linked polylactic acid in the polymer in addition to poor drawingability.

Example 6-2

Polylactic acid was synthesized by the conventional method using tinoctylate as a polymerizing catalyst with a starting material ratio of95.7 mol % of L-lactide and 4.3 mol % of D-lactide.

The polymer obtained had a relative viscosity of 3.3, weight averagemolecular weight Mw of 174,000, number average molecular weight Mn of91,000, monomer content of 0.20% by weight or less, and Sn content of 16ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted in a single screw extruder at 190° C., andmelt-extruded from a circular die extruder having a diameter of 30 cmwith a lip gap of 1.0 mm, followed by solidification by cooling to forma raw sheet. This sheet was slit into 6 mm wide stripes, which weredrawn on a hot plate followed by annealing heat-treatment in a hot airstream. The first step drawing was performed on a hot plate at atemperature of 115° C. with a draw magnification factor of 5.5, and thesecond step drawing was performed on a hot plate at a temperature of120° C. with a draw magnification factor of 1.2, followed byheat-setting at 130° C. in a hot air stream with an annealing ratio of5%, thereby obtaining a flat yarn with an width of 3 mm and lineardensity of 890 dtex.

The flat yarn obtained had a contraction ratio of 4.3%, tensile strengthof 2.7 cN/dtex and elongation of 36.0%. The rate of decrease ofviscosity during melt-extrusion of 4% suggests a small amount ofdecomposition of the polymer to avoid troubles in forming the raw sheet.The contraction ratio of 5.0% or less hardly generates contraction byheat-treatment when the yarn is processed into woven and knit fabricswith no changes of feeling, which is suitable for practical application.Problems that the fabric becomes unusable by the heat-settingtemperature were also avoided. The tensile strength of 2.6 cN/dtex ormore hardly arises troubles in the processing steps to make the strengthof the final product sufficient without any practical problems. Theelongation of 40.0% or less was practically favorable.

Example 6-3

Polylactic aid was synthesized by the conventional method using tinoctylate as a polymerizing catalyst with a starting material ratio of98.5 mol % of L-lactide and 1.5 mol % of D-lactide.

The polymer obtained had a relative viscosity of 4.2, weight averagemolecular weight Mw of 201,000, number average molecular weight Mn of103,000, monomer content of 0.20% by weight or less and Sn content of 16ppm with a heat stability temperature (5%) of 319° C.

The polymer was melted in a single screw extruder at 190° C., andmelt-extruded from a circular die extruder having a diameter of 30 cmwith a lip gap of 1.0 mm, followed by solidification by cooling to forma raw sheet. This sheet was slit into 6 mm wide stripes, which weredrawn on a hot plate followed by annealing heat-treatment in a hot airstream. The first step drawing was performed on a hot plate at atemperature of 118° C. with a draw magnification factor of 5.5, and thesecond step drawing was performed on a hot plate at a temperature of120° C. with a draw magnification factor of 1.2, followed byheat-setting at 130° C. in a hot air stream with an annealing ratio of5%, thereby obtaining a flat yarn with an width of 3 mm and lineardensity of 890 dtex.

The flat yarn obtained had a contraction ratio of 1.9%, tensile strengthof 3.4 cN/dtex and elongation of 30.0%. The rate of decrease ofviscosity during melt-extrusion of 4% suggests a small amount ofdecomposition of the polymer to avoid troubles in forming the raw sheet.

The contraction ratio of 5.0% or less hardly generates contraction byheat-treatment when the yarn is processed into woven and knit fabricswith no changes of feeling, which is suitable for practical application.Problems that the fabric becomes unusable by the heat-settingtemperature were also avoided. The tensile strength of 2.6 cN/dtex ormore hardly arises troubles in the processing steps to make the strengthof the final product sufficient without any practical problems. Theelongation of 40.0% or less was practically favorable.

(False-Twist Yarn and Producing Process Thereof)

The false-twist yarn and producing process thereof will be describedhereinafter.

A long term operation is difficult in the false-twist yarn manufacturedfrom a biodegradable resin currently known in the art because break ofyarns during processing frequently happens. Moreover, the tensilestrength and expansion-contraction recovery ratio are so low that crimpcharacteristics required for the false-twist yarn is extremely poor. Itis also a problem that a high quality fabric cannot be constantlysupplied due to frequently occurring break of yarns and fluffs in thepost processing such as weave and knit processing.

The inventors of the present invention have invented false-twist yarnsexcellent in work efficiency and properties by using polylactic acidhaving selected properties through intensive studies of the propertiesof polylactic acid as a starting material of the false-twist yarn. Theobject of the present invention is to provide a practically applicablefalse-twist yarn comprising polylactic acid with excellent workefficiency, wherein the polylactic acid fiber is capable of processinginto a twist yarn, wherein the polylactic acid twist yarn is free frombreak of yarns and filament with excellent characteristics as textiles,and wherein the twist yarn has physical properties such as tensilestrength and expansion/contraction recovery ratio comparative to thoseof conventional polyester twist yarns, and is to provide the processesfor producing thereof.

The false-twist yarn according to the present invention satisfies thefollowing features.

In a first aspect, the present invention provides a false-twist yarnmainly comprising a polylactic acid resin, wherein the monomer contentin the polylactic acid is 0.5% by weight or less.

In a second aspect according to the more preferred embodiment of thefirst aspect, the polylactic acid false-twist yarn comprises 95 mol % ormore of the L-isomer of the polylactic acid resin.

In a more preferable third aspect, the polylactic acid false-twist yarnaccording to the first and second aspects comprises a linear polylacticacid resin.

In a further preferable fourth aspect, the polylactic acid false-twistyarn according to the first to third aspects comprises the polylacticacid resin with ηrel of 2.7 to 3.9.

In a more preferable fifth aspect, the polylactic acid false-twist yarnaccording to the first to fourth aspect comprises the polylactic acidresin with an Sn content of 0 or 30 ppm or less.

In a more preferable sixth aspect, the polylactic acid false-twist yarnaccording to the first to fifth aspects has a tensile strength of 2.4cN/dtex or more.

In a more preferable seventh aspect, the polylactic acid false twistyarn according to the first to sixth aspects has a expansion/contractionrecovery ratio of 10% or more.

In the process for producing the polylactic acid false-twist yarn asdescribed above, a polylactic acid non-drawn yarn is subjected to asimultaneous draw and false-twist processing at a draw temperature of110° C. or more and draw magnification factor of 1.3 to 1.8, wherein thepolylactic acid resin according to the first to fifth aspects hasbirefringence Δn of 0.010 to 0.035, the tensile strength S (cN/dtex) andultimate elongation E (%) is represented by the relation of 15≦S×√E≦23.

The monomer content in polylactic acid according to the presentinvention is required to be 0 or 0.5% by weight or less. Monomers asdetermined in the present invention refers to the component having amolecular weight of 1,000 or less as determined by a GPC assay. Yarnsare liable to be fragile and the twisted yarn suffers extreme stresswhen the monomer content exceeds 0.5% by weight, thereby the tensilestrength is markedly decreases. Throughput of twist works turn out to beunstable due to frequent break of yarns during the process by the samereason as described above.

Usually, the reaction vessel is evacuated immediately before completingthe polymerization reaction for removing unreacted monomers in thepolylactic acid. Otherwise, polymerized chips may be washed with anappropriate solvent, or subjected to a solid state polymerization.

Lactic acid according to the present invention comprises naturallyoccurring L-lactic acid and D-lactic acid as an optical isomer ofL-lactic acid, L-lactide and D-lactide as dimers thereof, andmesolactide. The proportion of L-isomer is preferably 95 mol % or more,more preferably 98 mol % or more.

When the proportion of the L-isomer is 95 mol % or more, the resinbecomes highly heat resistant to allow the tensile strength of the yarnto be seldom decreased even by heat-setting at a relatively hightemperature. Heat-setting at a high temperature makesexpansion/contraction recovery ratio of the yarn to be excellent toenable a false-twist yarn with excellent crimp characteristics to beobtained.

The polylactic acid is preferably a linear polymer, or substantially hasno branched structure. Adding a branching agent in the polymerizationprocess of polylactic acid has been proposed for improving meltviscosity and degree of polymerization. However, it was confirmed by theinventors of the present invention that the branched structure of thepolylactic acid composition far more negatively affects properties ofthe false-twist yarn and work efficiency of the yarn as compared withconventional polyesters. In other words, the multifilament comprisingpolylactic acid having no branched structure seldom arises break ofyarns during false-twisting, and the false-twist yarn obtained therefromhas a higher tensile strength than the false-twist yarn having somebranched structure.

For excluding the branched structure, it is recommended to avoid use ofagents that arise the branched structure, for example three valent orfour valent alcohols and carboxylic acids, in the polymer material.However, when these chemicals are forced to use for some other reasons,the amount of use should be restricted within a range as small aspossible so that false-twist efficiency is not adversely affected.

Polylactic acid according to the present invention preferably has arelative viscosity (ηrel) of 2.7 to 3.9, because an excellentfalse-twist yarn may be obtained, or decrease of the tensile strength issuppressed to be minimum to decrease break of yarns during thefalse-twist process in this viscosity range.

The Sn content in polylactic acid according to the present invention ispreferably 0 or 30 ppm or less. While the Sn based catalyst is used as apolymerization catalyst of polylactic acid, an Sn content of 30 ppm orless permits decrease of the tensile strength to be suppressed to itsminimum besides decreasing the incidence of break of yarns in thefalse-twist process.

Although polylactic acid without the properties as described above orcommon resins other than polylactic acid may be used as startingmaterials in the false-twist yarn according to the present invention,the material is preferably a biodegradable resin such as an aliphaticpolyester for manufacturing a biodegradable false-twist yarn.

The false-twist yarn preferably has a tensile strength of 2.5 cN/dtex ormore, because incidence of break of yarns and fluffs decrease in thepost-processing such as weave and knit process when the tensile strengthfalls within the range above.

The false-twist yarn according to the present invention preferably has acontraction ratio in boiling water of 5% or more from the view point ofpreventing wrinkles from generating. The contraction ratio in boilingwater of 5% or more can prevent wrinkles from generating when fabricsare subjected to dyeing process.

The contraction ratio in boiling water is preferably 15% or less whenthe strength of the yarn is emphasized. The tensile strength and tearstrength may be secured without largely changing dimensions and mass perunit area of the fabric when contraction ratio in boiling water is 15%or less.

A contraction ratio in boiling water of 5 to 15% is preferable forsatisfying both prevention of wrinkles and retention of strength.

The false-twist yarn according to the present invention preferably has aexpansion/contraction recovery ratio of 10% or more, because the fabricis endowed with flexibility to enable the yarns to be developed in theapplication fields in which stretching properties are required. Moreovercrimp characteristics of the false-twist yarn permits fabrics having afluffy feeling to be supplied.

Commonly available false-twisting machines may be used for false-twistof the raw thread of the false-twist yarn comprising threads ofpolylactic acid. While the false-twisting machine is classified into across-belt type having a twist-rotor comprising a rubber based material,a pin-type having a twist-rotor comprising a metal, and a friction typefor twisting with a disk, the type of the machine is not particularlyrestricted.

The temperature of the plate heater for heat-setting is preferably 110to 150° C., more preferably 120 to 140° C. Since the melting point ofpolylactic acid is 170° C., molecular orientation is not disturbed at150° C. or less to enable the tensile strength to be avoided fromlargely decreased. A sufficient heat-setting is possible, on the otherhand, at 110° C. or more to make the expansion/contraction ratio to behigh to enable a false-twist yarn having excellent crimp characteristicsto be obtained.

EXAMPLES

The present invention will be described in detail with reference toexamples. While analysis processes of the physical and chemicalproperties of the polymer are described herein, those not describedbelow have been already described.

(Tensile Strength)

A load was applied to the sample by hanging a (indicated lineardensity×1/10) grams of weight. The sample with a length of 20 cm wasdrawn at a speed of 20 cm/min using a Tensiron type tensile strengthtester, and the tensile strength was calculated from the break forceusing the following equation:tensile strength (cN/dtex)=break force/actual linear density(Ultimate Elongation)

A load was applied to the sample by hanging a (indicated linear densityx×1/10) grams of weight. The sample with a chuck distance of 50 cm wasdrawn at a speed of 50 cm/min using an Instron type tensile strengthtester to measure the chuck distance (L) when the sample is broken, andthe ultimate elongation was calculated from the following equation:Ultimate elongation (%)=(L−50)/50×100(Contraction Ratio in Boiling Water)

A load was applied to the sample by hanging a (indicated linear densityx×1/10) grams of weight using a round scale with a frame circumferenceof 100 cm. A sub-reel with a reel number of ten was manufactured, andthe sample was immersed in water at room temperature by loading with an(indicated linear density× 1/10×20) grams weight to measure the lengthof the sample eight minutes after immersion. The sample was then takenout of water, folded twice as a figure of 8 and immersed in boilingwater for 80 minutes. The sample was again loaded with an (indicatedlinear density× 1/10×20) grams weight in water to measure the lengtheight minutes after immersion. The contraction ratio in boiling waterwas calculated by the following equation:Contraction ratio in boiling water (%)=[(initial sample length−samplelength after contraction)/initial sample length]×100(Expansion/Contraction Recovery Ratio)

A load was applied to the sample by hanging a (indicated linear density×1/10) grams of weight. A sub-reel with a reel number of ten wasmanufactured, and the sample was immersed in water at 20±2° C. for 3minutes by loading with an (indicated linear density× 1/10×20) gramsweight. The length (a) of the reel was at first measured and, afterallowing to stand for two minutes by removing the load, the length (b)of the reel was measured again to calculate the recovery ratio from thefollowing equation:Expansion/contraction recovery ratio (%)=(a−b)/a×100(Work Efficiency of False-Twist)

Work efficiency of false-twist was totally evaluated by the followingcriteria:

-   -   ⊚: incidence of break of yarns is one time/day or less among 48        spindles;    -   o: incidence of break of yarns is two to five times/day among 48        spindles;    -   Δ: incidence of break of yarns is six to 15 times/day among 48        spindles; and    -   x: incidence of break of yarns is 16 times/day or more among 48        spindles.        (Work Efficiency of Weaving)

Work efficiency of weaving when the yarn was woven using WJL was totallyevaluated by the following criteria:

-   -   ⊚: incidence of break of yarns is zero time a day;    -   o: incidence of break of yarns is one to two times a day;    -   Δ: incidence of break of yarns is three to nine times a day; and    -   x: incidence of break of yarns is ten times or more a day.        (Feeling of Textile)

Feeling of textile was totally evaluated by the following criteria:

-   -   ⊚: fluffy feeling of the textile is nearly identical to the        textile using regular polyester yarns;    -   o: fluffy feeling of the textile is somewhat inferior to the        textile using regular polyester yarns;    -   Δ: the textile using the false-twist yarn has somewhat better        fluffy feeling than the textile using the original yarn; and    -   x: there is no fluffy feeling at all.

Example 7-1

A false-twist yarn with a tensile strength of 3.2 cN/dtex andexpansion/contract recovery ratio of 16.4% was obtained from thepolylactic acid fibers having the composition shown in Table 7-1 byheat-setting at 130° C. using a false-twisting machine 33H-Mach Crimper(made by Murata Machine Co.) comprising a cross-belt type twist roller.Work efficiency of the yarn was favorable, and no break of yarns wasobserved after processing of 1 ton of yarns. When a textile was wovenwith a water-jet loom using this false-twist warn as a woof, fabricshaving sufficient fluffy feeling can be manufactured with substantiallyno break of yarns.

Example 7-2

A false-twist yarn with a tensile strength of 2.9 cN/dtex andexpansion/contract recovery ratio of 14.8% was obtained from thepolylactic acid fibers having the composition shown in Table 7-1 byheat-setting at 130° C. using a false-twisting machine ST-5 (made byMitsubishi Industrial Machine Co.) comprising a pin type twist roller.Work efficiency of the yarn relatively was favorable, and no break ofyarns was observed after processing of 1 ton of yarns. When a textilewas woven with a water-jet loom using this false-twist warn as a woof,fabrics having sufficient fluffy feeling can be manufactured withsubstantially no break of yarns.

Comparative Example 7-1

A false-twist yarn with a tensile strength of 1.9 cN/dtex andexpansion/contract recovery ratio of 13.3% was obtained from thepolylactic acid fibers containing a large proportion of monomers using afalse-twisting machine 33H-Mach Crimper (made by Murata Machine Co.)comprising a cross-belt type twist roller. The tensile strength was lowdue to large content of the monomer, and work efficiency wasconsiderably poor with frequent occurrence of break of yarns when atextile was woven using this false-twist yarn as a woof with a water-jetloom.

Example 7-3

A false-twist yarn with a tensile strength of 1.2 cN/dtex andexpansion/contraction recovery ratio of 6.7% was obtained from apolylactic acid fiber containing a small proportion of the L-isomer asshown in Table 7-1 using the false-twisting machine used in ComparativeExample 7-1. The false-twist yarn had a little higher contraction ratioin boiling water and a little low work efficiency. However, break ofyarns was seldom observed when a fabric was woof using this false-twistyarn as a woof with a water-jet loom.

Example 7-4

A false-twist yarn with a tensile strength of 2.2 cN/dtex andexpansion/contraction recovery ratio of 13.1% was obtained from apolylactic acid fiber containing branched structures as shown in Table7-1 using the false-twisting machine used in Comparative Example 7-1.Although work efficiency was a little poor with a few times of break ofyarns since the tensile strength is inferior to the yarns having nobranched structure in Example 7-1, the expansion/contraction recoveryratio was as high as 10% or more. When a fabric was woven using thisfalse-twist yarn as a woof with a water-jet weave machine, a fluffyfabric could be manufactured with few frequency of break of yarns.

Example 7-5

A false-twist yarn with a tensile strength of 1.6 cN/dtex andexpansion/contraction recovery ratio of 14.5% was obtained from apolylactic acid fiber having a low relative viscosity as shown in Table7-1 using the false-twisting machine used in Comparative Example 7-1.Although work efficiency was a little poor with a few times of break ofyarns due to a little inferior tensile strength of this false-twist yarnto the false-twist yarn having a favorable relative viscosity in Example7-1, the contraction rate in boiling water was low andexpansion/contraction recovery ratio was high. When a fabric was wovenusing this false-twist yarn as a woof with a water-jet loom, a fluffyfabric could be manufactured with few frequency of break of yarns.

Example 7-6

A false-twist yarn with a tensile strength of 2.3 cN/dtex andexpansion/contraction recovery ratio of 13.3% was obtained from apolylactic acid fiber having a high relative viscosity as shown in Table7-1 using the false-twisting machine used in Comparative Example 7-1.Although work efficiency was a little poor with a few times of break ofyarns due to a little inferior tensile strength of this false-twist yarnto the false-twist yarn having a favorable relative viscosity in Example7-1, the contraction rate in boiling water was low andexpansion/contraction recovery ratio was high. When a fabric was wovenusing this false-twist yarn as a woof with a water-jet loom, a fluffyfabric could be manufactured with few frequency of break of yarns.

Example 7-7

A false-twist yarn with a tensile strength of 1.3 cN/dtex andexpansion/contraction recovery ratio of 12.8% was obtained from apolylactic acid fiber containing a large amount of Sn as shown in Table7-1 using the false-twisting machine used in Comparative Example 7-1.Although work efficiency was a little poor with a few times of break ofyarns due to a low tensile strength of this false-twist yarn as comparedwith the false-twist yarn containing a small amount of Sn in Example7-1, the contraction rate in boiling water was low andexpansion/contraction recovery ratio was high. When a fabric was wovenusing this false-twist yarn as a woof with a water-jet loom, a fluffyfabric could be manufactured with few frequency of break of yarns.

TABLE 7-1 Comparative No. 7- Example 1 Example 2 Example 1 Example 3Example 4 Example 5 Example 6 Example 7 Sn Content (ppm) 16 16 18 21 1916 15 62 Relative Viscosity 3.05 3.05 2.92 3.05 3.04 2.05 4.02 2.94 ofPolymer (ηrel) Monomer Content 0.24 0.24 1.02 0.27 0.26 0.25 0.24 0.24(% by weight Branched Non Non Non Non Yes Non Non Non structure L-isomer(mol %) 98.6 98.6 98.2 92.6 99.0 97.6 97.0 95.5 twist roller cross-beltpin cross-belt cross-belt cross-belt cross-belt cross-belt cross-beltplate heater 130 130 130 130 130 130 130 130 temperature (° C.) tensilestrength 3.17 2.91 1.85 1.23 2.20 1.59 2.29 1.32 (cN/dtex) ultimate 26.727.2 26.4 22.2 28.7 24.2 27.4 25.0 elongation (%) expansion/contraction16.4 14.8 13.3 6.7 13.1 14.5 13.3 12.8 recovery ratio (%) contractionratio 10.8 9.8 10.3 25.1 10.4 10.1 12.3 11.6 in boiling water (%) workefficiency ⊚ ◯ X Δ Δ Δ Δ Δ of false-twist work efficiency ⊚ ⊚ X ◯ ◯ ◯ ◯◯ of weaving feeling of fabric ⊚ ⊚ ◯ Δ ◯ ◯ ◯ ◯

The producing process according to the present invention will bedescribed hereinafter.

A highly oriented non-drawn polylactic acid fiber with a birefringence(Δn) of 0.010 to 0.035, and tensile strength S (cN/dtex) and ultimateelongation (%) in the range of the following equation should be used forthe false-twist yarn according to the present invention.15≦S×√E≦23

Since the polylactic acid fiber is inferior in heat resistance to othersynthetic fibers, at draw and twist processing filaments are melt-fusedin the polylactic acid non-drawn yarn with a birefringence (Δn) of lessthan 0.010 and S×√E of less than 15 to make processing unstable. In thepolylactic acid highly oriented non-drawn yarn with a birefringence (Δn)of exceeding 0.035 and S×√E of exceeding 23, yarns having desirableproperties cannot be obtained due to too high orientation.

The heater temperature for simultaneous draw-and-twist processing isrequired to be 110° C. or more. A temperature of less than 110° C. failsin obtaining a false-twist yarn having sufficient properties.

The draw magnification factor in the simultaneous draw-and-twistprocessing should be 1.3 to 1.8. Satisfactory properties cannot beobtained at a factor of less than 1.3, while a factor of exceeding 1.8arises break of yarns to fail in practical production.

While other polymers may be used together, a biodegradable polymermaterial should be used for manufacturing a biodegradable false-twistyarn.

EXAMPLES

Polymerization of Polymer

Polylactic acid was synthesized by the conventional process usingL-lactide and D-lactide as starting materials and tin octylate as apolymerization catalyst. For comparison, polylactic acid was alsosynthesized by adding 0.1 mol % of trimellitic acid as a cross-linkagent. The polymer obtained was further subjected to solid satepolymerization at 135° C. to reduce the content of residual monomers.However, solid state polymerization was omitted in a part of the samplesfor comparative purposes.

Examples 8-1 to 8-4, Comparative examples 8-1 to 8-10

Each polylactic acid was melted at a predetermined temperature and spunfrom nozzle holes with a diameter of 0.3 mm. After reeling at a spinningspeed of 3800 m/min, the filaments were simultaneously drawn andfalse-twisted to produce a false-twist yarn with a linear density of 84dtex/24 f. The simultaneous draw-and-false twist machine used was 33Hmach Crimper made by Murata Machine Co.

As shown in the date of the examples in Tables 8-1 to 8-4, thefalse-twist yarns produced under the conditions according to the presentinvention had splendid properties. On the contrary, as shown in thecomparative examples 8-1 to 8-7, the false twist yarns having sufficientproperties could not obtained from the non-drawn yarns with Δn, S and Eout of the range of the present invention.

TABLE 8-1 Comparative Example Example No. 8- 1 2 3 1 2 Sn Content (ppm)18 19 62 26 17 Relative Viscosity of Polymer (ηrel) 2.92 3.02 2.94 2.932.98 Monomer Content (% by weight) 3.46 0.98 0.24 0.26 0.25 Branchedstructure Non Non Non Non Non L-isomer (mol %) 99.0 98.5 98.7 98.7 98.6Spinning Temperature (° C.) 230 230 230 230 230 Rate of Decrease ofViscosity during 20.3 10.0 17.6 5.0 3.6 spinning (%) non-drawn tensilestrength (cN/dtex) 1.55 1.87 1.76 2.07 2.12 yarn ultimate elongation (%)62.3 60.3 61.7 61.6 59.6 Δn 0.007 0.008 0.009 0.013 0.015 S × √E 12.214.5 13.8 16.2 16.4 fluffs X X X ◯ ◯ false-twist draw false twistmagnification 1.5 1.5 1.5 1.5 1.5 yarn factor Heater Temperature (° C.)130 130 130 130 130 tensile strength (cN/dtex) 1.76 2.02 2.04 2.67 2.68contraction ratio in boiling water 10.6 11.2 10.8 9.8 9.8 (%)expansion/contraction recovery 10.2 11.5 11.8 13.6 14.1 ratio (%) fluffsX X X ◯ ◯

TABLE 8-2 Comparative Example No. 8- 4 5 6 7 8 9 10 Sn Content (ppm) 1918 20 16.0 16.0 16.0 16.0 Relative Viscosity of 3.04 2.58 4.02 3.04 3.033.03 3.03 Polymer (ηrel) Monomer Content (% by 0.26 0.25 0.24 0.26 0.260.26 0.26 weight) Branched structure Yes Non Non Non Non Non NonL-isomer (mol %) 99.0 98.7 99.0 94.7 98.9 98.9 98.9 Spinning Temperature(° C.) 230 230 245 230 230 230 230 Rate of Decrease of 6.0 8.0 15.0 5.04.0 4.0 4.0 Viscosity during spinning (%) non-drawn tensile strength1.89 1.76 1.88 1.88 2.26 2.26 2.26 yarn (cN/dtex) ultimate 59.0 60.061.0 58.0 59.7 59.7 59.7 elongation (%) Δn 0.009 0.008 0.008 0.008 0.0170.017 0.017 S × √E 14.5 13.6 14.6 14.3 17.4 17.4 17.4 fluffs ◯ X X ◯ ◯ ◯◯ draw false twist magnification 1.5 1.5 1.5 1.5 1.2 1.5 2.0 factorHeater Temperature (° C.) 130 130 130 130 130 105 130 false-twisttensile strength 2.06 1.92 1.96 2.24 2.29 2.28 2.20 yarn (cN/dtex)contraction ratio 10.6 9.8 9.8 20.4 9.8 13.6 9.6 in boiling water (%)expansion/contraction 10.6 13.0 13.4 14.4 12.4 8.4 12.4 recovery ratio(%) fluffs ◯ X X ◯ ◯ ◯ ◯

TABLE 8-3 Example No. 8- 3 4 Sn Content (ppm) 16 15 Relative Viscosityof Polymer (ηrel) 3.05 2.94 Monomer Content (% by weight) 0.15 0.13Branched structure Yes Yes L-isomer (mol %) 99.0 98.7 SpinningTemperature (° C.) 230 230 non-drawn yarn Rate of Decrease of Viscosityduring 5.2 5.0 spinning (%) (%) tensile strength (cN/dtex) 2.24 2.29ultimate elongation (%) 58.9 60.0 Δn 0.025 0.024 S × √E 17.2 17.7 fluffs◯ ◯ draw false twist magnification factor 1.5 1.5 Heater Temperature (°C.) 130 130 false-twist yarn tensile strength (cN/dtex) 2.69 2.63contraction ratio in boiling water (%) 10.6 10.8 expansion/contractionrecovery ratio 15.6 14.6 (%) fluffs ◯ ◯(Filament Nonwoven Fabric)

Finally, the filament nonwoven fabric according to the present inventionwill be described below.

The polylactic acid filament nonwoven fabric known in the art include afilament nonwoven fabric having no core-and-sheath structure in which apolymer prepared by cross-linking a polybutylene succinate polymersynthesized from 1,4-butanediol and succinic acid with urethane bonds isblended with polylactic acid as a binder resin. However, this polymercomposition has so poor compatibility that a filament nonwoven fabrichaving a sufficient tensile strength cannot be obtained.

The inventors of the present invention have strictly surveyed theproperties of the polylactic acid as a starting material of the textile,and invented a polylactic acid filament nonwoven fabric having physicalproperties such as tensile strength and expansion ratio comparable tothose of polyester, nylon and polypropylene fibers, by using polylacticacid with selected properties and having a core-and-sheath structure.

In a first aspect, the present invention provides a polylactic acidfilament nonwoven fabric mainly comprising polylactic acid (PLA) andhaving a core-and-sheath structure, wherein the core to sheath ratio is1:1 to 5:1 in area ratio, and the sheath component comprises polylacticacid having a lower melting point than the core component, or the sheathcomponent comprises a blend of polylactic acid and other biodegradablepolymers having a lower melting point than polylactic acid.

In a second aspect, the present invention provides a filament nonwovenfabric having a core-and-sheath structure, wherein (a) the corecomponent has a linear structure with a relative viscosity of 2.5 to 3.5and Sn content of 0 or 30 ppm or less, and polylactic acid contains 98mol % or more of the L-isomer, and (b) the sheath component has a linearstructure with a relative viscosity of 2.5 to 3.5 and Sn content of 0 or30 ppm or less, and comprises polylactic acid with 96 mol % or less ofthe L-isomer and the core to sheath ratio of 1:1 to 5:1 in area ratio.

In a third aspect, the present invention provides a filament nonwovenfabric having a core-and-sheath structure, wherein (a) the corecomponent has a linear structure with a relative viscosity of 2.5 to 3.5and Sn content of 0 or 30 ppm or less, and polylactic acid contains 98mol % or more of the L-isomer, and (b) the sheath component has a linearstructure with a relative viscosity of 2.5 to 3.5 and Sn content of 0 or30 ppm or less, and comprises a blend of polylactic acid with 98 mol %or more of the L-isomer and a polymer prepared by cross-linking apolybutylene succinate polymer synthesized from 1,4-butanediol andsuccinic acid with urethane bonds, the weight ratio of polylactic acidbeing 50 to 90% and the core to sheath ratio being 1:1 to 5:1 in arearatio.

In a more preferable embodiment of the present invention, the polylacticacid filament nonwoven fabric has a mean linear density of 1 to 15 dtex,mass per unit area of 10 to 200 g/m² and tensile strength in thelongitudinal direction of 30N or more.

The first aspect according to the present invention will be describedfirst. In this aspect, polylactic acid is used for the core, andpolylactic acid having a lower melting point than the core component ora blend of a biodegradable polymer having a lower melting point than thepolylactic acid with polylactic acid is used for the sheath component.The core to sheath ratio is 1:1 to 5:1 in area ratio.

Forming the core-and-sheath structure allows polylactic acid crystal asthe core component to be fully oriented, and using polylactic acidhaving a lower melting point than the core component or a blend of abiodegradable polymer having a lower melting point than the polylacticacid with polylactic acid gives an advantage that filaments arepartially fused with each other so that a sufficiently high tensilestrength is obtained.

The core-and-sheath fiber according to the present invention is requiredto have a core to sheath ratio of 1:1 to 5:1. The proportion of thesheath component higher than this range is inadequate, since the tensilestrength may become insufficient and the fiber may adhere to the hotroller to decrease work efficiency. The proportion of the core componenthigher than this range is also inadequate, since the tensile strengthmay decrease due to insufficient partial fusion among the filaments orfluffs may appear in the filament nonwoven fabric.

The second aspect of the present invention will be describedhereinafter. The polylactic acid to be used in the present invention hasa linear structure, or substantially has no branched structure. It hasbeen proposed to add a small amount of a branching agent in preparingpolylactic acid in order to improve melt viscosity and degree ofpolymerization. However, it was confirmed by the inventors of thepresent invention that the branched structure of the polylactic acidcomposition far more negatively affects work efficiency of spinning ascompared with conventional polyesters. In other words, even a smallproportion of the branched structure in polylactic acid reduces thetensile strength as compared with polylactic acid having no branchedstructure.

For excluding the branched structure, it is recommended to avoid use ofagents that arise the branched structure, for example three valent orfour valent alcohols and carboxylic acids, in the polymer material.However, when such agent is forced to use for some reasons, the amountshould be restricted within a minimum essential range that does notaffect work efficiency of spinning such as break of fibers duringspinning.

The Sn content in polylactic acid to be used in the present invention is30 ppm or less, preferably 0 or 20 ppm or less. While the Sn basedcatalyst is used as the polymerization catalyst of polylactic acid, Sncontent exceeding 30 ppm induces depolymerization during spinning toextremely reduce work efficiency of spinning.

For reducing the Sn content, the amount of Sn to be used forpolymerization may be reduced, or the polymerized chips are washed withan appropriate solvent.

The polylactic acid to be used in the present invention has a relativeviscosity (ηrel) of 2.7 to 3.9. A viscosity lower than this rangereduces heat resistance of the polymer to make it impossible to attain asufficient tensile strength, while the higher viscosity forces thespinning temperature to be elevated to cause heat degradation duringspinning. Therefore, the preferable range is 2.7 to 3.0.

While polylactic acid to be used for the core component mainly comprisesL-lactic acid or D-lactic acid, L-lactide or D-lactide as a dimer oflactic acid, or mesolactide, it is crucial that the proportion of theL-isomer is 98 mol % or more. When the proportion of the L-isomer islower than 98 mol % crystal orientation during the producing process isinhibited from advancing to deteriorate the physical properties of thefibers obtained. The tensile strength is particularly reduced to makethe fibers practically inapplicable.

Polylactic acid to be used in the sheath component has a proportion ofthe L-isomer of 96 mol % or less in order to allow the sheath part tohave a different melting point from the melting point of the core part.The preferable proportion of the L-isomer is 91 to 95 mol %.

A polymer in which 10 to 50% by weight of a polymer, prepared bycross-linking a polybutylene succinate polymer synthesized from1,4-butanediol and succinic acid with urethane bonds and having a lowermelting point than L-lactic acid to be used for the core part, isblended with polylactic acid is preferably used for endowing the sheathpart with fusing property. A blend ratio of exceeding 50% makes fusingproperty among the filaments too high to make the nonwoven fabric toadhere on the hot roller, thereby making work efficiency andproductivity insufficient.

Various additives such as a lubricants, an oxidation stabilizer and heatstabilizer may be added, if necessary, to the polymer to be used in thepresent invention in the range not compromising the effect of thepresent invention.

It is essential that the core-to-sheath ratio is in the range of 1:1 to5:1 in area ratio. A larger proportion of the sheath component than thisrange is inappropriate, since the tensile strength may becomeinsufficient or the filament nonwoven fabric may fuse the hot roller toreduce work efficiency. A larger proportion of the core component isalso inappropriate, because filaments are not partially fused with eachother to reduce the tensile strength, or fluffs may appear in thefilament nonwoven fabric.

The filament nonwoven fabric according to the present inventionpreferably has a mean linear density of 1 to 15 dtex. When the lineardensity exceeds 15 dtex, cooling performance may be poor duringproducing, or flexibility of the filament nonwoven fabric may becompromised, thereby arising practical problems. The linear density ofless than 1 dtex may reduce productivity due to frequent occurrence ofbreak of fibers.

The third aspect of the present invention will be described hereinafter.The same quality of polylactic acid as used in the second aspect of thepresent invention should be used in this aspect.

The polymer for blend to be used in the sheath component according tothe present invention is a polymer prepared by cross-linkingpolybutylene succinate polymer synthesized from 1,4-butanediol andsuccinic acid with urethane bonds.

For blending the polymer with polylactic acid to form a sheathcomponent, the required blending ratio of polylactic acid is 50 to 90%by weight. When the proportion of polylactic acid is less than 50% byweight, filaments are too strongly fused with each other to form asheet, or the filament nonwoven fabric is fused on the hot roller toreduce productivity. When the proportion of polylactic acid exceeds 90%by weight, on the other hand, fluffs may appear due to insufficientfusion among the filaments with a low tensile strength to make thefabric to be practically inapplicable.

The required core-to-sheath ratio in the present invention is 1:1 to 5:1in area ratio. A larger proportion of the sheath component than thisrange is not appropriate, since the tensile strength may becomeinsufficient or the filament nonwoven fabric may fuse the hot roller toreduce work efficiency. A larger proportion of the core component isalso inappropriate since partial fusion among the filaments is not sosufficient that the tensile strength becomes insufficient, or fluffs mayappear in the filament nonwoven fabric.

The filament nonwoven fabrics according to the three aspects of thepresent invention as described above preferably have a mean lineardensity of 1 to 15 dtex, mass per unit area of 10 to 200 g/m² andlongitudinal tensile strength of 30N or more. A linear density in thisrange permits sufficient productivity to be obtained. A mass per unitarea in this range makes the fabric flexible, while a longitudinaltensile strength in this range arises no troubles in respectiveprocessing steps.

The producing process of the filament nonwoven fabric comprises thesteps of, for example, dispersing the filaments while drawing by reelingthem at a reel speed of 3,000 m/min to 6,000 m/min, collecting andpiling the filaments on a moving support made of a capture wire nets,and partially fusing the filaments on a roll at a roll temperature of100 to 150° C. to obtain a filament nonwoven fabric.

The reel speed in this is preferable since crystal orientationsufficiently advances to enhance work efficiency.

The roll temperature is preferably 100 to 150° C. A temperature ofhigher than 150° C. is too close to the melting point of polylactic acidof the core component that the nonwoven fabric fuses on the roller toarise problems in productivity.

EXAMPLES

The present invention will be described in more detail hereinafter withreference to examples. The analysis method of physical and chemicalproperties of the polymer will be described first. The method notdescribed herein has been hitherto described.

(Measurement of Elongation Percentage)

A sample piece with a dimension of about 5 cm×20 cm was extracted from asample. After attaching the sample piece to a tensile strength testerwith a chuck distance of 10 cm, the sample piece was drawn at a drawspeed of 20 cm/min to measure the load (N) at break of the sample piece.

Spinning work efficiency was measured and evaluated as follows:

(evaluation of productivity)

-   -   o: productivity is very excellent with good spinning ability and        hot-roll passing performance; and    -   x: continuous production is impossible due to poor spinning        ability and hot-roll passing performance.

Examples 9-1 to 9-3

The filaments were spun at a spinning temperature of 230° C., reeled ata reel speed of 3,000 m/min, and captured and piled on a moving wirecapture support in Examples and Comparative Examples. The capturedfilaments were processed into a filament nonwoven fabric with a meanlinear density of 2.2 dtex and mass per unit area of 30 g/m² at a rolltemperature of 145° C.

TABLE 9-1 Example Comparative Example No. 9- 1 2 3 1 2 3 4core-to-sheath area 1:1 2:1 5:1 1:1 7:1 1:3 2:1 ratio L-isomer (%) incore 98.4 99.2 98.7 98.4 98.4 98.4 98.4 PLA core PLA melting 170 172 171170 170 170 170 point (° C.) L-isomer (%) in sheath 94.0 92.0 94.0 97.093.0 93.2 92.8 PLA sheath PLA melting 140 128 140 168 135 138 128 point(° C.) Relative viscosity 3.0 2.6 3.2 2.9 2.7 3.1 2.9 ηrel content ofresidual Sn 17 20 13 16 21 13 12 (ppm) branched structure Non Non NonNon Non Non Yes longitudinal tensile 77.4 87.2 94.1 26.5 29.4 18.4 25.6strength (N) productivity ◯ ◯ ◯ X X X X

TABLE 9-2 Comparative Example No. 9- 5 6 7 8 core-to-sheath area ratio2:1 1:1 1:1 1:1 L-isomer (%) in core PLA 98.3 98.6 98.6 93.8 core PLAmelting point (° C.) 170 170 171 140 L-isomer (%) in sheath PLA 93.794.2 93.8 98.6 sheath PLA melting point (° C.) 140 141 140 171 Relativeviscosity of ηrel 2.9 2.3 3.7 2.9 content of residual Sn (ppm) 70 17 1616 branched structure Non Non Non Non longitudinal tensile strength (N)19.6 22.5 24.5 19.5 productivity X X X X

Tables 9-1 and 9-2 show that the filament nonwoven fabric obtainedwithin the conditions of the present invention is excellent in physicalproperties such as the tensile strength and productivity.

The sample in Comparative Example 9-1 contained a larger proportion ofthe L-isomer, filaments were not partially fused with each other byhot-rolling, and a lot of fluffs were generated. The sample inComparative Example 9-2 having a small area ratio of the sheath part wasalso absent in partial fusion among the filaments, while the sample inComparative Example 9-3 was, on the contrary, had a too large area ratioof the sheath part that the nonwoven fabric fused on the hot-roll.

The sample in Comparative Example 9-4 in which a branched polymer wasused could not attain a sufficient tensile strength due to the branchedstructure.

The sample in Comparative Example 9-5 containing a large amount ofresidual Sn caused depolymerization during spinning to extremely reducespinning work efficiency.

The sample in Comparative Example 9-6 having a lower polymer viscosityfailed in obtaining a sufficient tensile strength, while the sample inComparative Example 9-7 having a higher polymer viscosity was forced toelevate the spinning temperature to cause heat decomposition of thepolymer during spinning, thereby making it impossible to obtain afilament nonwoven fabric having a sufficient tensile strength.

A polymer having a higher melting point is used in the sheath componentin Comparative Example 9-8. The filaments were not partially fused byhot rolling due to the high melting point of the sheath component togenerate fluff in the filament spun-bond fabric, thereby causing poorproductivity. Consequently, a filament nonwoven fabric having asufficient tensile strength could not be obtained.

TABLE 9-3 Comparative Example Example No. 9- 4 5 8 9 core-to-sheath arearatio 1:1 2:1 1:1 1:1 L-isomer (%) in core PLA 98.3 98.6 98.5 98.6 blendratio of the sheath (%) 20 40 5 70 Relative viscosity ηrel 3.1 2.9 2.92.8 content of residual Sn (ppm) 13 18 13 16 branched structure Non NonNon Non longitudinal tensile strength (N) 84.2 88.2 15.6 — productivity◯ ◯ X X

The blend ration of the polymer (trade name: Bionole, melting point 110°C.) as a sheath component, prepared by cross-linking a polybutylenesuccinate polymer synthesized from 1,4-butanediol and succinic acid byurethane bonds, is changed as shown in Table 9-3. While there were noproblems in the blend ratio within the range of the present invention(Examples 9-4 and 9-5), the nonwoven fabric was fused on the hot-roll tomake production impossible in the Comparative Example 9-9 in which theblending ratio was increased. In Comparative Example 9-8 in which theblending ratio was reduced, on the other hand, the filaments were notpartially fused with each other to create fluffs in the nonwoven fabric.

INDUSTRIAL APPLICABILITY

The present invention provides a textile product being excellent in workefficiency and having excellent properties of the fiber comprisingpolylactic acid that is free from practical problems for industrialproduction, and a process for producing the textile product.

1. A polylactic acid resin comprising a linear polylactic acid with arelative viscosity ηrel of in the range of 2.7 to 3.9, prepared fromlactic acid monomers wherein at least 95 mol % of the lactic acid is anL-isomer, and wherein the resin contains 0 to 30 ppm of tin (Sn) and 0to 0.5% by weight of residual monomer.
 2. A polylactic acid fibercomprising the polylactic acid resin according to claim
 1. 3. A processfor producing a polylactic acid fiber by melt-spinning the polylacticacid according to claim
 1. 4. A polylactic acid fiber comprising thepolylactic acid resin according to claim 1, wherein the polylactic acidfiber is a staple fiber.
 5. A polylactic acid fiber according to claim 4having a tensile strength of 2.6 cN/dtex or more, an elongation of 80%or less, a contraction ratio in boiling water of 5.0% or less and acrimp number in the range of 4 to 18 crimps/25 mm.
 6. A binder fibercomprising a polylactic acid resin comprising a linear polylactic acidwith a relative viscosity ηrel of in the range of 2.7 to 3.9, preparedfrom lactic acid monomers wherein at least 90 mol % of the lactic acidis an L-isomer, and wherein the resin contains 0 to 30 ppm of Sn and 0to 0.5% by weight of residual monomer.
 7. The binder fiber according toclaim 6 having a structure with a core and a sheath, wherein the corecontains a polylactic acid resin wherein at least 98 mol % of the lacticacid is an L-isomer, and the sheath contains a polylactic acid resinwherein at least 90 mol % of the lactic acid is an L-isomer.
 8. Thebinder fiber according to claim 7 having a structure with a core and asheath, wherein the proportion C (mol %) of L-isomer in polylactic acidof the core and the proportion S (mol %) of L-isomer in the polylacticacid of the sheath satisfies the relation: 2≦C−S≦8.
 9. The binder fiberaccording to claim 7 having a tensile strength of 2.6 cN/dtex or more,an elongation of 80% or less, a heat-contraction ratio at 80° C. of15.0% or less, and a crimp number in the range of 4 to 18 crimps/25 mm.10. A process for producing a polylactic acid binder fiber according toclaim 6 comprising the steps: spinning at a spinning temperature in therange of 210° C. to 240° C. and spinning speed in the range of 600 to1,200 m/min, drawing at a draw magnification factor in the range of 3.0to 5.0 at a draw temperature in the range of 40° C. to 70° C., andheat-treating at a temperature in the range of 60° C. to 90° C.